On Thu 18-12-25 09:40:36, Jinchao Wang wrote:
On Wed, Dec 17, 2025 at 12:30:15PM +0100, Jan Kara wrote:
Hello!
On Tue 16-12-25 19:34:55, Jinchao Wang wrote:
syzbot reported a KASAN out-of-bounds Read in ext4_xattr_set_entry()[1].
When xattr_find_entry() returns -ENODATA, search.here still points to the position after the last valid entry. ext4_xattr_block_set() clones the xattr block because the original block maybe shared and must not be modified in place.
In the clone_block, search.here is recomputed unconditionally from the old offset, which may place it past search.first. This results in a negative reset size and an out-of-bounds memmove() in ext4_xattr_set_entry().
Fix this by initializing search.here correctly when search.not_found is set.
[1] https://syzkaller.appspot.com/bug?extid=f792df426ff0f5ceb8d1
Fixes: fd48e9acdf2 (ext4: Unindent codeblock in ext4_xattr_block_set) Cc: stable@vger.kernel.org Reported-by: syzbot+f792df426ff0f5ceb8d1@syzkaller.appspotmail.com Signed-off-by: Jinchao Wang wangjinchao600@gmail.com
Thanks for the patch! But I think the problem must be somewhere else.
The first syzbot test report was run without the patch applied, which caused confusion. The correct usage and report show that this patch fixes the crash: https://lore.kernel.org/all/20251216123945.391988-2-wangjinchao600@gmail.com... https://lore.kernel.org/all/6941580e.a70a0220.33cd7b.013d.GAE@google.com/
I was not arguing that your patch doesn't fix this syzbot issue. Just that I don't understand how what you describe can happen and thus I'm not sure whether the fix is really the best one...
in ext4_xattr_set_entry(). And I don't see how 'here' can be greater than 'last' which should be pointing to the very same 4-byte zeroed word. The fact that 'here' and 'last' are not equal is IMO the problem which needs debugging and it indicates there's something really fishy going on with the xattr block we work with. The block should be freshly allocated one as far as I'm checking the disk image (as the 'file1' file doesn't have xattr block in the original image).
I traced the crash path and find how this hapens:
Thanks for sharing the details!
entry_SYSCALL_64 ... ext4_xattr_move_to_block ext4_xattr_block_find (){ error = xattr_find_entry(inode, &bs->s.here, ...); // bs->s.here updated // to ENTRY(header(s->first)+1); if (error && error != -ENODATA) return error; bs->s.not_found = error; // and returned to the caller } ext4_xattr_block_set (bs) { s = bs->s; offset = (char *)s->here - bs->bh->b_data; // bs->bh->b_data == bs->s.base // offset = ENTRY(header(s->first)+1) - s.base // leads to wrong offset
Why do you think the offset is wrong here? The offset is correct AFAICS - it will be the offset of the 0 word from the beginning of xattr block. I have run the reproducer myself and as I guessed in my previous email the real problem is that someone modifies the xattr block between we compute the offset here and the moment we call kmemdup() in clone_block. Thus the computation of 'last' in ext4_xattr_set_entry() yields a different result that what we saw in ext4_xattr_block_set(). The block modification happens because the xattr block - block 33 is used for it - is also referenced from file3 (but it was marked as unused in the block bitmap and so xattr block got placed there).
So your patch was fixing the problem only by chance and slightly different syzbot reproducer (overwriting the block 33 with a different contents) would trigger the crash again.
So far I wasn't able to figure out how exactly the block 33 got zeroed out but with corrupted filesystem it can happen in principle rather easily. The question is how we can possibly fix this because this is one of the nastier cases of fs corrution to deal with. The overhead of re-verifying fs metadata each time we relock the buffer is just too big... So far no great ideas for this.
Honza
clone_block: {s->base = kmemdup(BHDR(bs->bh), bs->bh->b_size, GFP_NOFS); s->first = ENTRY(header(s->base)+1); s->here = ENTRY(s->base + offset); // wrong s->here } ext4_xattr_set_entry (s) { last = s->first; // last < here rest = (void *)last - (void *)here + sizeof(__u32); // negative rest memmove((void *)here + size, here, rest); // crash } }
Honzadiff --git a/fs/ext4/xattr.c b/fs/ext4/xattr.c index 2e02efbddaac..cc30abeb7f30 100644 --- a/fs/ext4/xattr.c +++ b/fs/ext4/xattr.c @@ -1980,7 +1980,10 @@ ext4_xattr_block_set(handle_t *handle, struct inode *inode, goto cleanup; s->first = ENTRY(header(s->base)+1); header(s->base)->h_refcount = cpu_to_le32(1);
s->here = ENTRY(s->base + offset);
if (s->not_found)s->here = s->first;elses->here = ENTRY(s->base + offset);/* -- 2.43.0
-- Jan Kara jack@suse.com SUSE Labs, CR