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Message-ID: <qn7ncujf5gkfmohf5qp3fdakrymhoapkscafqp5t2gulmgdqai@tuhu2igx33k4>
Date: Thu, 20 Mar 2025 20:02:34 +0000
From: Pedro Falcato <pfalcato@...e.de>
To: syzbot <syzbot+20ed41006cf9d842c2b5@...kaller.appspotmail.com>,
lorenzo.stoakes@...cle.com
Cc: Liam.Howlett@...cle.com, akpm@...ux-foundation.org, jannh@...gle.com,
linux-kernel@...r.kernel.org, linux-mm@...ck.org, syzkaller-bugs@...glegroups.com,
vbabka@...e.cz
Subject: Re: [syzbot] [mm?] BUG: unable to handle kernel paging request in
vma_merge_existing_range
On Thu, Mar 20, 2025 at 12:09:36PM -0700, syzbot wrote:
> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit: eb88e6bfbc0a Merge tag 'fsnotify_for_v6.14-rc7' of git://g..
> git tree: upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=11e6c83f980000
> kernel config: https://syzkaller.appspot.com/x/.config?x=77423669c2b8fa9
> dashboard link: https://syzkaller.appspot.com/bug?extid=20ed41006cf9d842c2b5
> compiler: gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
> userspace arch: i386
>
> Unfortunately, I don't have any reproducer for this issue yet.
>
> Downloadable assets:
> disk image (non-bootable): https://storage.googleapis.com/syzbot-assets/7feb34a89c2a/non_bootable_disk-eb88e6bf.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/ded0ce69669f/vmlinux-eb88e6bf.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/6e6fa3c719e7/bzImage-eb88e6bf.xz
>
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+20ed41006cf9d842c2b5@...kaller.appspotmail.com
>
> RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000293 R12: 0000000000000000
> R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
> </TASK>
> BUG: unable to handle page fault for address: fffffffffffffff4
> #PF: supervisor read access in kernel mode
> #PF: error_code(0x0000) - not-present page
> PGD df84067 P4D df84067 PUD df86067 PMD 0
> Oops: Oops: 0000 [#1] PREEMPT SMP KASAN NOPTI
> CPU: 1 UID: 0 PID: 17805 Comm: syz.8.3237 Not tainted 6.14.0-rc6-syzkaller-00212-geb88e6bfbc0a #0
> Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2~bpo12+1 04/01/2014
> RIP: 0010:vma_merge_existing_range+0x266/0x2070 mm/vma.c:734
> Code: e8 5f 25 ad ff 48 8b 14 24 48 b8 00 00 00 00 00 fc ff df 48 c1 ea 03 80 3c 02 00 0f 85 1c 19 00 00 48 8b 04 24 48 8b 74 24 08 <4c> 8b 38 4c 89 ff e8 9f 1f ad ff 48 8b 44 24 08 49 39 c7 0f 83 db
> RSP: 0000:ffffc9000319f988 EFLAGS: 00010246
> RAX: fffffffffffffff4 RBX: ffffc9000319fae8 RCX: ffffffff820cd3e5
> RDX: 1ffffffffffffffe RSI: 0000000080c2a000 RDI: 0000000000000005
> RBP: 0000000080ce2000 R08: 0000000000000005 R09: 0000000000000000
> R10: 0000000000000001 R11: 0000000000000001 R12: 0000000000000001
> R13: ffffc9000319fb08 R14: ffff888025eddc98 R15: ffff88804eec0a00
> FS: 0000000000000000(0000) GS:ffff88802b500000(0063) knlGS:00000000f5106b40
> CS: 0010 DS: 002b ES: 002b CR0: 0000000080050033
> CR2: fffffffffffffff4 CR3: 00000000614d6000 CR4: 0000000000352ef0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> Call Trace:
> <TASK>
> vma_modify.constprop.0+0x87/0x410 mm/vma.c:1517
> vma_modify_flags_uffd+0x241/0x2e0 mm/vma.c:1598
> userfaultfd_clear_vma+0x91/0x130 mm/userfaultfd.c:1906
> userfaultfd_release_all+0x2ae/0x4c0 mm/userfaultfd.c:2024
> userfaultfd_release+0xf4/0x1c0 fs/userfaultfd.c:865
> __fput+0x3ff/0xb70 fs/file_table.c:464
> task_work_run+0x14e/0x250 kernel/task_work.c:227
> resume_user_mode_work include/linux/resume_user_mode.h:50 [inline]
> exit_to_user_mode_loop kernel/entry/common.c:114 [inline]
> exit_to_user_mode_prepare include/linux/entry-common.h:329 [inline]
> __syscall_exit_to_user_mode_work kernel/entry/common.c:207 [inline]
> syscall_exit_to_user_mode+0x27b/0x2a0 kernel/entry/common.c:218
> __do_fast_syscall_32+0x80/0x120 arch/x86/entry/common.c:390
> do_fast_syscall_32+0x32/0x80 arch/x86/entry/common.c:412
> entry_SYSENTER_compat_after_hwframe+0x84/0x8e
> RIP: 0023:0xf7fe6579
> Code: b8 01 10 06 03 74 b4 01 10 07 03 74 b0 01 10 08 03 74 d8 01 00 00 00 00 00 00 00 00 00 00 00 00 00 51 52 55 89 e5 0f 34 cd 80 <5d> 5a 59 c3 90 90 90 90 8d b4 26 00 00 00 00 8d b4 26 00 00 00 00
> RSP: 002b:00000000f510655c EFLAGS: 00000296 ORIG_RAX: 0000000000000135
> RAX: 0000000000000001 RBX: 0000000080000180 RCX: 0000000000000001
> RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
> RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000293 R12: 0000000000000000
> R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
> </TASK>
> Modules linked in:
> CR2: fffffffffffffff4
> ---[ end trace 0000000000000000 ]---
> RIP: 0010:vma_merge_existing_range+0x266/0x2070 mm/vma.c:734
> Code: e8 5f 25 ad ff 48 8b 14 24 48 b8 00 00 00 00 00 fc ff df 48 c1 ea 03 80 3c 02 00 0f 85 1c 19 00 00 48 8b 04 24 48 8b 74 24 08 <4c> 8b 38 4c 89 ff e8 9f 1f ad ff 48 8b 44 24 08 49 39 c7 0f 83 db
> RSP: 0000:ffffc9000319f988 EFLAGS: 00010246
> RAX: fffffffffffffff4 RBX: ffffc9000319fae8 RCX: ffffffff820cd3e5
> RDX: 1ffffffffffffffe RSI: 0000000080c2a000 RDI: 0000000000000005
> RBP: 0000000080ce2000 R08: 0000000000000005 R09: 0000000000000000
> R10: 0000000000000001 R11: 0000000000000001 R12: 0000000000000001
> R13: ffffc9000319fb08 R14: ffff888025eddc98 R15: ffff88804eec0a00
> FS: 0000000000000000(0000) GS:ffff88802b500000(0063) knlGS:00000000f5106b40
> CS: 0010 DS: 002b ES: 002b CR0: 0000000080050033
> CR2: fffffffffffffff4 CR3: 00000000614d6000 CR4: 0000000000352ef0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> ----------------
> Code disassembly (best guess):
> 0: e8 5f 25 ad ff call 0xffad2564
> 5: 48 8b 14 24 mov (%rsp),%rdx
> 9: 48 b8 00 00 00 00 00 movabs $0xdffffc0000000000,%rax
> 10: fc ff df
> 13: 48 c1 ea 03 shr $0x3,%rdx
> 17: 80 3c 02 00 cmpb $0x0,(%rdx,%rax,1)
> 1b: 0f 85 1c 19 00 00 jne 0x193d
> 21: 48 8b 04 24 mov (%rsp),%rax
> 25: 48 8b 74 24 08 mov 0x8(%rsp),%rsi
> * 2a: 4c 8b 38 mov (%rax),%r15 <-- trapping instruction
> 2d: 4c 89 ff mov %r15,%rdi
> 30: e8 9f 1f ad ff call 0xffad1fd4
> 35: 48 8b 44 24 08 mov 0x8(%rsp),%rax
> 3a: 49 39 c7 cmp %rax,%r15
> 3d: 0f .byte 0xf
> 3e: 83 .byte 0x83
> 3f: db .byte 0xdb
Ahh, fun bug. This *seems* to be the bug:
First, in vma_modify:
merged = vma_merge_existing_range(vmg);
if (merged)
return merged;
if (vmg_nomem(vmg))
return ERR_PTR(-ENOMEM);
then, all the way up to userfaultfd_release_all (the return value propagates
vma_modify -> vma_modify_flags_uffd -> userfaultfd_clear_vma):
prev = NULL;
for_each_vma(vmi, vma) {
cond_resched();
BUG_ON(!!vma->vm_userfaultfd_ctx.ctx ^
!!(vma->vm_flags & __VM_UFFD_FLAGS));
if (vma->vm_userfaultfd_ctx.ctx != ctx) {
prev = vma;
continue;
}
vma = userfaultfd_clear_vma(&vmi, prev, vma,
vma->vm_start, vma->vm_end);
prev = vma;
}
So, if uffd gets an IS_ERR(vma), it keeps going and takes that vma as the prev value,
which leads to that ERR_PTR(-ENOMEM) deref crash (-12 = -ENOMEM = 0xffffff4).
This situation is kind of awkward because ->release() errors don't mean a thing.
So, I have another idea (pasting for syzbot) which might just be cromulent.
Untested, but thoughts?
#syz test
diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index d06453fa8aba..fb835d82eb84 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -2023,6 +2023,8 @@ void userfaultfd_release_all(struct mm_struct *mm,
vma = userfaultfd_clear_vma(&vmi, prev, vma,
vma->vm_start, vma->vm_end);
+ if (WARN_ON(IS_ERR(vma)))
+ break;
prev = vma;
}
mmap_write_unlock(mm);
diff --git a/mm/vma.c b/mm/vma.c
index 71ca012c616c..b2167b7dc27d 100644
--- a/mm/vma.c
+++ b/mm/vma.c
@@ -1517,8 +1517,16 @@ static struct vm_area_struct *vma_modify(struct vma_merge_struct *vmg)
merged = vma_merge_existing_range(vmg);
if (merged)
return merged;
- if (vmg_nomem(vmg))
+ if (vmg_nomem(vmg)) {
+ /* If we can avoid failing the whole modification
+ * due to a merge OOM and validly keep going
+ * (we're modifying the whole VMA), return vma intact.
+ * It won't get merged, but such is life - we're avoiding
+ * OOM conditions in other parts of mm/ this way */
+ if (start <= vma->vm_start && end >= vma->vm_end)
+ return vma;
return ERR_PTR(-ENOMEM);
+ }
/* Split any preceding portion of the VMA. */
if (vma->vm_start < start) {
--
Pedro
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