Re: [PATCH] 9pfs: fix crash on 'Treaddir' request

[Christian Schoenebeck <qemu_oss@crudebyte.com>]

On Tuesday, November 5, 2024 11:25:26 AM CET Christian Schoenebeck wrote:
> A bad (broken or malicious) 9p client (guest) could cause QEMU host to
> crash by sending a 9p 'Treaddir' request with a numeric file ID (FID) that
> was previously opened for a file instead of an expected directory:
> 
>   #0  0x0000762aff8f4919 in __GI___rewinddir (dirp=0xf) at
>     ../sysdeps/unix/sysv/linux/rewinddir.c:29
>   #1  0x0000557b7625fb40 in do_readdir_many (pdu=0x557bb67d2eb0,
>     fidp=0x557bb67955b0, entries=0x762afe9fff58, offset=0, maxsize=131072,
>     dostat=<optimized out>) at ../hw/9pfs/codir.c:101
>   #2  v9fs_co_readdir_many (pdu=pdu@entry=0x557bb67d2eb0,
>     fidp=fidp@entry=0x557bb67955b0, entries=entries@entry=0x762afe9fff58,
>     offset=0, maxsize=131072, dostat=false) at ../hw/9pfs/codir.c:226
>   #3  0x0000557b7625c1f9 in v9fs_do_readdir (pdu=0x557bb67d2eb0,
>     fidp=0x557bb67955b0, offset=<optimized out>,
>     max_count=<optimized out>) at ../hw/9pfs/9p.c:2488
>   #4  v9fs_readdir (opaque=0x557bb67d2eb0) at ../hw/9pfs/9p.c:2602
> 
> That's because V9fsFidOpenState was declared as union type. So the
> same memory region is used for either an open POSIX file handle (int),
> or a POSIX DIR* pointer, etc., so 9p server incorrectly used the
> previously opened (valid) POSIX file handle (0xf) as DIR* pointer,
> eventually causing a crash in glibc's rewinddir() function.
> 
> Root cause was therefore a missing check in 9p server's 'Treaddir'
> request handler, which must ensure that the client supplied FID was
> really opened as directory stream before trying to access the
> aforementioned union and its DIR* member.
> 
> Cc: qemu-stable@nongnu.org
> Fixes: d62dbb51f7 ("virtio-9p: Add fidtype so that we can do type ...")
> Reported-by: Akihiro Suda <suda.kyoto@gmail.com>
> Tested-by: Akihiro Suda <suda.kyoto@gmail.com>
> Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>

Queued on 9p.next:
https://github.com/cschoenebeck/qemu/commits/9p.next

I'll send a PR tomorrow.

Thanks!

/Christian