linux-kernel - Re: BUG: unable to handle kernel paging request in fuse_copy

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Date: Fri, 22 Mar 2024 16:41:17 +0100
From: David Hildenbrand <david@...hat.com>
To: Miklos Szeredi <miklos@...redi.hu>, xingwei lee <xrivendell7@...il.com>
Cc: linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
 samsun1006219@...il.com, syzkaller-bugs@...glegroups.com,
 linux-mm <linux-mm@...ck.org>, Mike Rapoport <rppt@...nel.org>
Subject: Re: BUG: unable to handle kernel paging request in fuse_copy_do

On 22.03.24 14:50, Miklos Szeredi wrote:
> [MM list + secretmem author CC-d]
> 
> On Thu, 21 Mar 2024 at 08:52, xingwei lee <xrivendell7@...il.com> wrote:
>>
>> Hello I found a bug titled "BUG: unable to handle kernel paging
>> request in fuse_copy_do” with modified syzkaller, and maybe it is
>> related to fs/fuse.
>> I also confirmed in the latest upstream.
>>
>> If you fix this issue, please add the following tag to the commit:
>> Reported-by: xingwei lee <xrivendell7@...il.com>
>> Reported-by: yue sun <samsun1006219@...il.com>
> 
> Thanks for the report.   This looks like a secretmem vs get_user_pages issue.
> 
> I reduced the syz reproducer to a minimal one that isn't dependent on fuse:
> 
> === repro.c ===
> #define _GNU_SOURCE
> 
> #include <fcntl.h>
> #include <unistd.h>
> #include <sys/mman.h>
> #include <sys/syscall.h>
> #include <sys/socket.h>
> 
> int main(void)
> {
>          int fd1, fd2, fd3;
>          int pip[2];
>          struct iovec iov;
>          void *addr;
> 
>          fd1 = syscall(__NR_memfd_secret, 0);
>          addr = mmap(NULL, 4096, PROT_READ, MAP_SHARED, fd1, 0);
>          ftruncate(fd1, 7);
>          fd2 = socket(AF_INET, SOCK_DGRAM, 0);
>          getsockopt(fd2, 0, 0, NULL, addr);
> 
>          pipe(pip);
>          iov.iov_base = addr;
>          iov.iov_len = 0x50;
>          vmsplice(pip[1], &iov, 1, 0);

pip[1] should be the write end. So it will be used as the source.

I assume we go the ITER_SOURCE path in vmsplice, and call 
vmsplice_to_pipe(). Then we call iter_to_pipe().

I would expect iov_iter_get_pages2() -> get_user_pages_fast() to fail on 
secretmem pages?

But at least the vmsplice() just seems to work. Which is weird, because 
GUP-fast should not apply (page not faulted in?) and check_vma_flags() 
bails out early on vma_is_secretmem(vma).

So something is not quite right.

> 
>          fd3 = open("/tmp/repro-secretmem.test", O_RDWR | O_CREAT, 0x600);
>          splice(pip[0], NULL, fd3, NULL, 0x50, 0);
> 
>          return 0;
> }




-- 
Cheers,

David / dhildenb