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

[Pierrick Bouvier]

Hi Volker,

On 12/29/24 01:24, Volker Rümelin wrote:
> > Found with test sbsaref introduced in [1].
> >
> > [1] 
> > 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 .

Thanks very much for analyzing and fixing the underlying issue Volker.

I was not sure what was happening here, but doing memory aliasing with 
two different structs is definitely a suspicious pattern.
I'll test the patch you sent, and give a review once I check it works.

Regards,
Pierrick

> 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) {