Re: [PULL 04/11] vvfat: fix ubsan issue in create_long_filename

[Volker Rümelin]

> Found with test sbsaref introduced in [1].
>
> [1] 
> 20241203213629.2482806-1-pierrick.bouvier@linaro.org/">https://patchew.org/QEMU/20241203213629.2482806-1-pierrick.bouvier@linaro.org/
>
> ../block/vvfat.c:433:24: runtime error: index 14 out of bounds for type 
> 'uint8_t [11]'
>     #0 0x56151a66b93a in create_long_filename ../block/vvfat.c:433
>     #1 0x56151a66f3d7 in create_short_and_long_name ../block/vvfat.c:725
>     #2 0x56151a670403 in read_directory ../block/vvfat.c:804
>     #3 0x56151a674432 in init_directories ../block/vvfat.c:964
>     #4 0x56151a67867b in vvfat_open ../block/vvfat.c:1258
>     #5 0x56151a3b8e19 in bdrv_open_driver ../block.c:1660
>     #6 0x56151a3bb666 in bdrv_open_common ../block.c:1985
>     #7 0x56151a3cadb9 in bdrv_open_inherit ../block.c:4153
>     #8 0x56151a3c8850 in bdrv_open_child_bs ../block.c:3731
>     #9 0x56151a3ca832 in bdrv_open_inherit ../block.c:4098
>     #10 0x56151a3cbe40 in bdrv_open ../block.c:4248
>     #11 0x56151a46344f in blk_new_open ../block/block-backend.c:457
>     #12 0x56151a388bd9 in blockdev_init ../blockdev.c:612
>     #13 0x56151a38ab2d in drive_new ../blockdev.c:1006
>     #14 0x5615190fca41 in drive_init_func ../system/vl.c:649
>     #15 0x56151aa796dd in qemu_opts_foreach ../util/qemu-option.c:1135
>     #16 0x5615190fd2b6 in configure_blockdev ../system/vl.c:708
>     #17 0x56151910a307 in qemu_create_early_backends ../system/vl.c:2004
>     #18 0x561519113fcf in qemu_init ../system/vl.c:3685
>     #19 0x56151a7e438e in main ../system/main.c:47
>     #20 0x7f72d1a46249 in __libc_start_call_main 
> ../sysdeps/nptl/libc_start_call_main.h:58
>     #21 0x7f72d1a46304 in __libc_start_main_impl ../csu/libc-start.c:360
>     #22 0x561517e98510 in _start 
> (/home/user/.work/qemu/build/qemu-system-aarch64+0x3b9b510)
>
> The offset used can easily go beyond entry->name size. It's probably a
> bug, but I don't have the time to dive into vfat specifics for now.

Hi Pierrick, Michael,

this patch breaks the creation of long filenames in the vvfat driver.

> This change solves the ubsan issue, and is functionally equivalent, as
> anything written past the entry->name array would not be read anyway.

This assumption is wrong. The guest reads the bytes written past the
entry->name array in the 32 byte direntry_t structure. A LFN direntry
structure is different from a regular direntry structure.

You patch limits the long file name to 5 UCS-2 characters out of
possible 13 UCS-2 characters per LFN entry.

To reproduce the issue:

On the host:
~> mkdir vvfat-drive
~> touch "vvfat-drive/file with a long name.txt"

and start QEMU with -blockdev
driver=vvfat,read-only=true,dir=./vvfat-drive,node-name=disk-d,label=hostdrive
-device scsi-hd,bus=scsi0.0,scsi-id=0,lun=1,drive=disk-d

On the guest without this patch:
~ # mount -t vfat -o ro /dev/sdb1 /mnt
~ # ls /mnt
file with a long name.txt

On the guest with this patch:
~ # mount -t vfat -o ro /dev/sdb1 /mnt
~ # ls /mnt
file
~ # ls /mnt | xxd
00000000: 6669 6c65 200a                           file .

With best regards,
Volker

>
> Signed-off-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
> Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
> Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
> ---
>  block/vvfat.c | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/block/vvfat.c b/block/vvfat.c
> index 8ffe8b3b9b..f2eafaa923 100644
> --- a/block/vvfat.c
> +++ b/block/vvfat.c
> @@ -426,6 +426,10 @@ static direntry_t *create_long_filename(BDRVVVFATState 
> *s, const char *filename)
>          else if(offset<22) offset=14+offset-10;
>          else offset=28+offset-22;
>          entry=array_get(&(s->directory),s->directory.next-1-(i/26));
> +        /* ensure we don't write anything past entry->name */
> +        if (offset >= sizeof(entry->name)) {
> +            continue;
> +        }
>          if (i >= 2 * length + 2) {
>              entry->name[offset] = 0xff;
>          } else if (i % 2 == 0) {