TLDRVuln - Cybersecurity Vulnerabilities Made Simple

In the Linux kernel, the following vulnerability has been resolved:

ext2/dax: Fix ext2_setsize when len is page aligned

PAGE_ALIGN(x) macro gives the next highest value which is multiple of

pagesize. But if x is already page aligned then it simply returns x.

So, if x passed is 0 in dax_zero_range() function, that means the

length gets passed as 0 to ->iomap_begin().

In ext2 it then calls ext2_get_blocks -> max_blocks as 0 and hits bug_on

here in ext2_get_blocks().

BUG_ON(maxblocks == 0);

Instead we should be calling dax_truncate_page() here which takes

care of it. i.e. it only calls dax_zero_range if the offset is not

page/block aligned.

This can be easily triggered with following on fsdax mounted pmem

device.

dd if=/dev/zero of=file count=1 bs=512

truncate -s 0 file

[79.525838] EXT2-fs (pmem0): DAX enabled. Warning: EXPERIMENTAL, use at your own risk

[79.529376] ext2 filesystem being mounted at /mnt1/test supports timestamps until 2038 (0x7fffffff)

[93.793207] ------------[ cut here ]------------

[93.795102] kernel BUG at fs/ext2/inode.c:637!

[93.796904] invalid opcode: 0000 [#1] PREEMPT SMP PTI

[93.798659] CPU: 0 PID: 1192 Comm: truncate Not tainted 6.3.0-rc2-xfstests-00056-g131086faa369 #139

[93.806459] RIP: 0010:ext2_get_blocks.constprop.0+0x524/0x610

<...>

[93.835298] Call Trace:

[93.836253] <TASK>

[93.837103] ? lock_acquire+0xf8/0x110

[93.838479] ? d_lookup+0x69/0xd0

[93.839779] ext2_iomap_begin+0xa7/0x1c0

[93.841154] iomap_iter+0xc7/0x150

[93.842425] dax_zero_range+0x6e/0xa0

[93.843813] ext2_setsize+0x176/0x1b0

[93.845164] ext2_setattr+0x151/0x200

[93.846467] notify_change+0x341/0x4e0

[93.847805] ? lock_acquire+0xf8/0x110

[93.849143] ? do_truncate+0x74/0xe0

[93.850452] ? do_truncate+0x84/0xe0

[93.851739] do_truncate+0x84/0xe0

[93.852974] do_sys_ftruncate+0x2b4/0x2f0

[93.854404] do_syscall_64+0x3f/0x90

[93.855789] entry_SYSCALL_64_after_hwframe+0x72/0xdc