Date:   Thu, 30 Jun 2022 09:14:07 -0700
From:   Axel Rasmussen <axelrasmussen@...gle.com>
To:     Mike Rapoport <rppt@...nel.org>
Cc:     Eric Biggers <ebiggers@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>, syzkaller-bugs@...glegroups.com,
        syzbot <syzbot+9bd2b7adbd34b30b87e4@...kaller.appspotmail.com>,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [syzbot] BUG: unable to handle kernel paging request in truncate_inode_partial_folio

On Thu, Jun 30, 2022 at 1:47 AM Mike Rapoport <rppt@...nel.org> wrote:
>
> On Wed, Jun 29, 2022 at 09:30:12AM -0700, Axel Rasmussen wrote:
> > On Tue, Jun 28, 2022 at 9:41 PM Eric Biggers <ebiggers@...nel.org> wrote:
> > >
> > > On Tue, Jun 28, 2022 at 03:59:26PM -0700, syzbot wrote:
> > > > Hello,
> > > >
> > > > syzbot found the following issue on:
> > > >
> > > > HEAD commit:    941e3e791269 Merge tag 'for_linus' of git://git.kernel.org..
> > > > git tree:       upstream
> > > > console+strace: https://syzkaller.appspot.com/x/log.txt?x=1670ded4080000
> > > > kernel config:  https://syzkaller.appspot.com/x/.config?x=833001d0819ddbc9
> > > > dashboard link: https://syzkaller.appspot.com/bug?extid=9bd2b7adbd34b30b87e4
> > > > compiler:       gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2
> > > > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=140f9ba8080000
> > > > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=15495188080000
> > > >
> > > > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > > > Reported-by: syzbot+9bd2b7adbd34b30b87e4@...kaller.appspotmail.com
> > > >
> > > > BUG: unable to handle page fault for address: ffff888021f7e005
> > > > #PF: supervisor write access in kernel mode
> > > > #PF: error_code(0x0002) - not-present page
> > > > PGD 11401067 P4D 11401067 PUD 11402067 PMD 21f7d063 PTE 800fffffde081060
> > > > Oops: 0002 [#1] PREEMPT SMP KASAN
> > > > CPU: 0 PID: 3761 Comm: syz-executor281 Not tainted 5.19.0-rc4-syzkaller-00014-g941e3e791269 #0
> > > > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> > > > RIP: 0010:memset_erms+0x9/0x10 arch/x86/lib/memset_64.S:64
> > > > Code: c1 e9 03 40 0f b6 f6 48 b8 01 01 01 01 01 01 01 01 48 0f af c6 f3 48 ab 89 d1 f3 aa 4c 89 c8 c3 90 49 89 f9 40 88 f0 48 89 d1 <f3> aa 4c 89 c8 c3 90 49 89 fa 40 0f b6 ce 48 b8 01 01 01 01 01 01
> > > > RSP: 0018:ffffc9000329fa90 EFLAGS: 00010202
> > > > RAX: 0000000000000000 RBX: 0000000000001000 RCX: 0000000000000ffb
> > > > RDX: 0000000000000ffb RSI: 0000000000000000 RDI: ffff888021f7e005
> > > > RBP: ffffea000087df80 R08: 0000000000000001 R09: ffff888021f7e005
> > > > R10: ffffed10043efdff R11: 0000000000000000 R12: 0000000000000005
> > > > R13: 0000000000000000 R14: 0000000000001000 R15: 0000000000000ffb
> > > > FS:  00007fb29d8b2700(0000) GS:ffff8880b9a00000(0000) knlGS:0000000000000000
> > > > CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > > > CR2: ffff888021f7e005 CR3: 0000000026e7b000 CR4: 00000000003506f0
> > > > DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> > > > DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> > > > Call Trace:
> > > >  <TASK>
> > > >  zero_user_segments include/linux/highmem.h:272 [inline]
> > > >  folio_zero_range include/linux/highmem.h:428 [inline]
> > > >  truncate_inode_partial_folio+0x76a/0xdf0 mm/truncate.c:237
> > > >  truncate_inode_pages_range+0x83b/0x1530 mm/truncate.c:381
> > > >  truncate_inode_pages mm/truncate.c:452 [inline]
> > > >  truncate_pagecache+0x63/0x90 mm/truncate.c:753
> > > >  simple_setattr+0xed/0x110 fs/libfs.c:535
> > > >  secretmem_setattr+0xae/0xf0 mm/secretmem.c:170
> > > >  notify_change+0xb8c/0x12b0 fs/attr.c:424
> > > >  do_truncate+0x13c/0x200 fs/open.c:65
> > > >  do_sys_ftruncate+0x536/0x730 fs/open.c:193
> > > >  do_syscall_x64 arch/x86/entry/common.c:50 [inline]
> > > >  do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
> > > >  entry_SYSCALL_64_after_hwframe+0x46/0xb0
> > > > RIP: 0033:0x7fb29d900899
> > > > Code: 28 00 00 00 75 05 48 83 c4 28 c3 e8 11 15 00 00 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 b8 ff ff ff f7 d8 64 89 01 48
> > > > RSP: 002b:00007fb29d8b2318 EFLAGS: 00000246 ORIG_RAX: 000000000000004d
> > > > RAX: ffffffffffffffda RBX: 00007fb29d988408 RCX: 00007fb29d900899
> > > > RDX: 00007fb29d900899 RSI: 0000000000000005 RDI: 0000000000000003
> > > > RBP: 00007fb29d988400 R08: 0000000000000000 R09: 0000000000000000
> > > > R10: 0000000000000000 R11: 0000000000000246 R12: 00007fb29d98840c
> > > > R13: 00007ffca01a23bf R14: 00007fb29d8b2400 R15: 0000000000022000
> > > >  </TASK>
> > > > Modules linked in:
> > > > CR2: ffff888021f7e005
> > > > ---[ end trace 0000000000000000 ]---
> > >
> > > I think this is a bug in memfd_secret.  secretmem_setattr() can race with a page
> > > being faulted in by secretmem_fault().  Specifically, a page can be faulted in
> > > after secretmem_setattr() has set i_size but before it zeroes out the partial
> > > page past i_size.  memfd_secret pages aren't mapped in the kernel direct map, so
> > > the crash occurs when the kernel tries to zero out the partial page.
> > >
> > > I don't know what the best solution is -- maybe a rw_semaphore protecting
> > > secretmem_fault() and secretmem_setattr()?  Or perhaps secretmem_setattr()
> > > should avoid the call to truncate_setsize() by not using simple_setattr(), given
> > > that secretmem_setattr() only supports the size going from zero to nonzero.
> >
> > From my perspective the rw_semaphore approach sounds reasonable.
> >
> > simple_setattr() and the functions it calls to do the actual work
> > isn't a tiny amount of code, it would be a shame to reimplement it in
> > secretmem.c.
> >
> > For the rwsem, I guess the idea is setattr will take it for write, and
> > fault will take it for read? Since setattr is a very infrequent
> > operation - a typical use case is you'd do it exactly once right after
> > opening the memfd_secret - this seems like it wouldn't make fault
> > significantly less performant. It's also a pretty small change I
> > think, just a few lines.
>
> Below is my take on adding a semaphore and making ->setattr() and ->fault()
> mutually exclusive. It's only lightly tested so I'd appreciate if Eric
> could give it a whirl.
>
> With addition of semaphore to secretmem_setattr() it seems we don't need
> special care for size changes, just calling simple_setattr() after taking
> the semaphore should be fine. Thoughts?

The patch below looks correct to me. I do think we still need the
check which prevents truncating a memfd_secret with an existing
nonzero size, though, because I think simple_setattr's way of doing
that still BUGs in a non-racy way (rwsem doesn't help with this). The
patch below keeps this, so maybe I'm just misinterpreting "we don't
need special care for size changes".

I haven't booted+tested it, I'll leave that to Eric since he already
has a reproducer setup for this. But, for what it's worth, feel free
to take:

Reviewed-by: Axel Rasmussen <axelrasmussen@...gle.com>


>
> From edfcb2f0d31c2132bda483635dd2a8dd295efb04 Mon Sep 17 00:00:00 2001
> From: Mike Rapoport <rppt@...ux.ibm.com>
> Date: Thu, 30 Jun 2022 11:26:37 +0300
> Subject: [PATCH] secretmem: fix unhandled fault in truncate
>
> syzkaller reports the following issue:
>
> BUG: unable to handle page fault for address: ffff888021f7e005
> PGD 11401067 P4D 11401067 PUD 11402067 PMD 21f7d063 PTE 800fffffde081060
> Oops: 0002 [#1] PREEMPT SMP KASAN
> CPU: 0 PID: 3761 Comm: syz-executor281 Not tainted 5.19.0-rc4-syzkaller-00014-g941e3e791269 #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> RIP: 0010:memset_erms+0x9/0x10 arch/x86/lib/memset_64.S:64
> Code: c1 e9 03 40 0f b6 f6 48 b8 01 01 01 01 01 01 01 01 48 0f af c6 f3 48 ab 89 d1 f3 aa 4c 89 c8 c3 90 49 89 f9 40 88 f0 48 89 d1 <f3> aa 4c 89 c8 c3 90 49 89 fa 40 0f b6 ce 48 b8 01 01 01 01 01 01
> RSP: 0018:ffffc9000329fa90 EFLAGS: 00010202
> RAX: 0000000000000000 RBX: 0000000000001000 RCX: 0000000000000ffb
> RDX: 0000000000000ffb RSI: 0000000000000000 RDI: ffff888021f7e005
> RBP: ffffea000087df80 R08: 0000000000000001 R09: ffff888021f7e005
> R10: ffffed10043efdff R11: 0000000000000000 R12: 0000000000000005
> R13: 0000000000000000 R14: 0000000000001000 R15: 0000000000000ffb
> FS:  00007fb29d8b2700(0000) GS:ffff8880b9a00000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: ffff888021f7e005 CR3: 0000000026e7b000 CR4: 00000000003506f0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> Call Trace:
>  <TASK>
>  zero_user_segments include/linux/highmem.h:272 [inline]
>  folio_zero_range include/linux/highmem.h:428 [inline]
>  truncate_inode_partial_folio+0x76a/0xdf0 mm/truncate.c:237
>  truncate_inode_pages_range+0x83b/0x1530 mm/truncate.c:381
>  truncate_inode_pages mm/truncate.c:452 [inline]
>  truncate_pagecache+0x63/0x90 mm/truncate.c:753
>  simple_setattr+0xed/0x110 fs/libfs.c:535
>  secretmem_setattr+0xae/0xf0 mm/secretmem.c:170
>  notify_change+0xb8c/0x12b0 fs/attr.c:424
>  do_truncate+0x13c/0x200 fs/open.c:65
>  do_sys_ftruncate+0x536/0x730 fs/open.c:193
>  do_syscall_x64 arch/x86/entry/common.c:50 [inline]
>  do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
>  entry_SYSCALL_64_after_hwframe+0x46/0xb0
> RIP: 0033:0x7fb29d900899
> Code: 28 00 00 00 75 05 48 83 c4 28 c3 e8 11 15 00 00 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 b8 ff ff ff f7 d8 64 89 01 48
> RSP: 002b:00007fb29d8b2318 EFLAGS: 00000246 ORIG_RAX: 000000000000004d
> RAX: ffffffffffffffda RBX: 00007fb29d988408 RCX: 00007fb29d900899
> RDX: 00007fb29d900899 RSI: 0000000000000005 RDI: 0000000000000003
> RBP: 00007fb29d988400 R08: 0000000000000000 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000246 R12: 00007fb29d98840c
> R13: 00007ffca01a23bf R14: 00007fb29d8b2400 R15: 0000000000022000
>  </TASK>
> Modules linked in:
> CR2: ffff888021f7e005
> ---[ end trace 0000000000000000 ]---
>
> Eric Biggers suggested that this happens when
> secretmem_setattr()->simple_setattr() races with secretmem_fault() so
> that a page that is faulted in by secretmem_fault() (and thus removed
> from the direct map) is zeroed by inode truncation right afterwards.
>
> Use an rw_semaphore to make secretmem_fault() and secretmem_setattr()
> mutually exclusive.
>
> Reported-by: syzbot+9bd2b7adbd34b30b87e4@...kaller.appspotmail.com
> Suggested-by: Eric Biggers <ebiggers@...nel.org>
> Signed-off-by: Mike Rapoport <rppt@...ux.ibm.com>
> ---
>  mm/secretmem.c | 48 ++++++++++++++++++++++++++++++++++++++++--------
>  1 file changed, 40 insertions(+), 8 deletions(-)
>
> diff --git a/mm/secretmem.c b/mm/secretmem.c
> index 206ed6b40c1d..40573b045c96 100644
> --- a/mm/secretmem.c
> +++ b/mm/secretmem.c
> @@ -47,30 +47,41 @@ bool secretmem_active(void)
>         return !!atomic_read(&secretmem_users);
>  }
>
> +struct secretmem_state {
> +       struct rw_semaphore rw_sem;
> +};
> +
>  static vm_fault_t secretmem_fault(struct vm_fault *vmf)
>  {
>         struct address_space *mapping = vmf->vma->vm_file->f_mapping;
>         struct inode *inode = file_inode(vmf->vma->vm_file);
> +       struct secretmem_state *state = inode->i_private;
>         pgoff_t offset = vmf->pgoff;
>         gfp_t gfp = vmf->gfp_mask;
>         unsigned long addr;
>         struct page *page;
> +       vm_fault_t ret;
>         int err;
>
>         if (((loff_t)vmf->pgoff << PAGE_SHIFT) >= i_size_read(inode))
>                 return vmf_error(-EINVAL);
>
> +       down_read(&state->rw_sem);
> +
>  retry:
>         page = find_lock_page(mapping, offset);
>         if (!page) {
>                 page = alloc_page(gfp | __GFP_ZERO);
> -               if (!page)
> -                       return VM_FAULT_OOM;
> +               if (!page) {
> +                       ret = VM_FAULT_OOM;
> +                       goto out;
> +               }
>
>                 err = set_direct_map_invalid_noflush(page);
>                 if (err) {
>                         put_page(page);
> -                       return vmf_error(err);
> +                       ret = vmf_error(err);
> +                       goto out;
>                 }
>
>                 __SetPageUptodate(page);
> @@ -86,7 +97,8 @@ static vm_fault_t secretmem_fault(struct vm_fault *vmf)
>                         if (err == -EEXIST)
>                                 goto retry;
>
> -                       return vmf_error(err);
> +                       ret = vmf_error(err);
> +                       goto out;
>                 }
>
>                 addr = (unsigned long)page_address(page);
> @@ -94,7 +106,11 @@ static vm_fault_t secretmem_fault(struct vm_fault *vmf)
>         }
>
>         vmf->page = page;
> -       return VM_FAULT_LOCKED;
> +       ret = VM_FAULT_LOCKED;
> +
> +out:
> +       up_read(&state->rw_sem);
> +       return ret;
>  }
>
>  static const struct vm_operations_struct secretmem_vm_ops = {
> @@ -163,11 +179,17 @@ static int secretmem_setattr(struct user_namespace *mnt_userns,
>  {
>         struct inode *inode = d_inode(dentry);
>         unsigned int ia_valid = iattr->ia_valid;
> +       struct secretmem_state *state = inode->i_private;
> +       int ret;
>
> +       down_write(&state->rw_sem);
>         if ((ia_valid & ATTR_SIZE) && inode->i_size)
> -               return -EINVAL;
> +               ret = -EINVAL;
> +       else
> +               ret = simple_setattr(mnt_userns, dentry, iattr);
> +       up_write(&state->rw_sem);
>
> -       return simple_setattr(mnt_userns, dentry, iattr);
> +       return ret;
>  }
>
>  static const struct inode_operations secretmem_iops = {
> @@ -179,22 +201,30 @@ static struct vfsmount *secretmem_mnt;
>  static struct file *secretmem_file_create(unsigned long flags)
>  {
>         struct file *file = ERR_PTR(-ENOMEM);
> +       struct secretmem_state *state;
>         struct inode *inode;
>
>         inode = alloc_anon_inode(secretmem_mnt->mnt_sb);
>         if (IS_ERR(inode))
>                 return ERR_CAST(inode);
>
> +       state = kzalloc(sizeof(*state), GFP_KERNEL);
> +       if (!state)
> +               goto err_free_inode;
> +
>         file = alloc_file_pseudo(inode, secretmem_mnt, "secretmem",
>                                  O_RDWR, &secretmem_fops);
>         if (IS_ERR(file))
> -               goto err_free_inode;
> +               goto err_free_state;
>
>         mapping_set_gfp_mask(inode->i_mapping, GFP_HIGHUSER);
>         mapping_set_unevictable(inode->i_mapping);
>
> +       init_rwsem(&state->rw_sem);
> +
>         inode->i_op = &secretmem_iops;
>         inode->i_mapping->a_ops = &secretmem_aops;
> +       inode->i_private = state;
>
>         /* pretend we are a normal file with zero size */
>         inode->i_mode |= S_IFREG;
> @@ -202,6 +232,8 @@ static struct file *secretmem_file_create(unsigned long flags)
>
>         return file;
>
> +err_free_state:
> +       kfree(state);
>  err_free_inode:
>         iput(inode);
>         return file;
>
> base-commit: 03c765b0e3b4cb5063276b086c76f7a612856a9a
> --
> 2.34.1
>
>
> > > The following commit tried to fix a similar bug, but it wasn't enough:
> > >
> > >         commit f9b141f93659e09a52e28791ccbaf69c273b8e92
> > >         Author: Axel Rasmussen <axelrasmussen@...gle.com>
> > >         Date:   Thu Apr 14 19:13:31 2022 -0700
> > >
> > >             mm/secretmem: fix panic when growing a memfd_secret
> > >
> > >
> > > Here's a simplified reproducer.  Note, for memfd_secret to be supported, the
> > > kernel config must contain CONFIG_SECRETMEM=y and the kernel command line must
> > > contain secretmem.enable=1.
> > >
> > > #include <pthread.h>
> > > #include <setjmp.h>
> > > #include <signal.h>
> > > #include <sys/mman.h>
> > > #include <sys/syscall.h>
> > > #include <unistd.h>
> > >
> > > static volatile int fd;
> > > static jmp_buf jump_buf;
> > >
> > > static void *truncate_thread(void *arg)
> > > {
> > >         for (;;)
> > >                 ftruncate(fd, 1000);
> > > }
> > >
> > > static void handle_sigbus(int sig)
> > > {
> > >         longjmp(jump_buf, 1);
> > > }
> > >
> > > int main(void)
> > > {
> > >         struct sigaction act = {
> > >                 .sa_handler = handle_sigbus,
> > >                 .sa_flags = SA_NODEFER,
> > >         };
> > >         pthread_t t;
> > >         void *addr;
> > >
> > >         sigaction(SIGBUS, &act, NULL);
> > >
> > >         pthread_create(&t, NULL, truncate_thread, NULL);
> > >         for (;;) {
> > >                 fd = syscall(__NR_memfd_secret, 0);
> > >                 addr = mmap(NULL, 8192, PROT_WRITE, MAP_SHARED, fd, 0);
> > >                 if (setjmp(jump_buf) == 0)
> > >                         *(unsigned int *)addr = 0;
> > >                 munmap(addr, 8192);
> > >                 close(fd);
> > >         }
> > > }
>
> --
> Sincerely yours,
> Mike.

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