On 9/20/23 5:41 AM, Eric Whitney wrote: > * Muhammad Usama Anjum <usama.anjum@xxxxxxxxxxxxx>: >> Syzbot has hit the following bug on current and all older kernels: >> BUG: KASAN: out-of-bounds in ext4_ext_rm_leaf fs/ext4/extents.c:2736 [inline] >> BUG: KASAN: out-of-bounds in ext4_ext_remove_space+0x2482/0x4d90 fs/ext4/extents.c:2958 >> Read of size 18446744073709551508 at addr ffff888073aea078 by task syz-executor420/6443 >> >> On investigation, I've found that eh->eh_entries is zero, ex is >> referring to last entry and EXT_LAST_EXTENT(eh) is referring to first. >> Hence EXT_LAST_EXTENT(eh) - ex becomes negative and causes the wrong >> buffer read. >> >> element: FFFF8882F8F0D06C <----- ex >> element: FFFF8882F8F0D060 >> element: FFFF8882F8F0D054 >> element: FFFF8882F8F0D048 >> element: FFFF8882F8F0D03C >> element: FFFF8882F8F0D030 >> element: FFFF8882F8F0D024 >> element: FFFF8882F8F0D018 >> element: FFFF8882F8F0D00C <------ EXT_FIRST_EXTENT(eh) >> header: FFFF8882F8F0D000 <------ EXT_LAST_EXTENT(eh) and eh >> >> Cc: stable@xxxxxxxxxxxxxxx >> Reported-by: syzbot+6e5f2db05775244c73b7@xxxxxxxxxxxxxxxxxxxxxxxxx >> Closes: https://groups.google.com/g/syzkaller-bugs/c/G6zS-LKgDW0/m/63MgF6V7BAAJ >> Fixes: d583fb87a3ff ("ext4: punch out extents") >> Signed-off-by: Muhammad Usama Anjum <usama.anjum@xxxxxxxxxxxxx> >> --- >> This patch is only fixing the local issue. There may be bigger bug. Why >> is ex set to last entry if the eh->eh_entries is 0. If any ext4 >> developer want to look at the bug, please don't hesitate. >> --- >> fs/ext4/extents.c | 2 +- >> 1 file changed, 1 insertion(+), 1 deletion(-) >> >> diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c >> index e4115d338f101..7b7779b4cb87f 100644 >> --- a/fs/ext4/extents.c >> +++ b/fs/ext4/extents.c >> @@ -2726,7 +2726,7 @@ ext4_ext_rm_leaf(handle_t *handle, struct inode *inode, >> * If the extent was completely released, >> * we need to remove it from the leaf >> */ >> - if (num == 0) { >> + if (num == 0 && eh->eh_entries) { >> if (end != EXT_MAX_BLOCKS - 1) { >> /* >> * For hole punching, we need to scoot all the >> -- >> 2.40.1 >> > > Hi: > > First, thanks for taking the time to look at this. Thank you for replying and giving me pointers that I need to start looking at problem from first warning until the bug which can be difficult until I debug the problem smartly and learn at least the basics of ext4. > > I'm suspicious that syzbot may be fuzzing an extent header or other extent > tree components. As you noticed, eh_entries and ex appear to be inconsistent. > Also, note the long series of corrupted file system reports in the console log > occurring before the KASAN bug - ext4 had been detecting and rejecting bad > data up to that point. The file system on the disk image provided by sysbot > indicates that metadata checksumming was enabled (and it fscks cleanly). > That should have caught a corrupted extent header or inode, but perhaps > there's a problem. > > The console log indicates that the problem occurred on inode #16. Does the > information you've provided above come from testing you did on inode #16 > (looks like the name was /bin/base64)? I couldn't analyze the problem in broad spectrum. There must be some bigger thing wrong here. > > By any chance, have you found a simpler reproducer than what syzbot provides? Not yet, this gets reproduced after a while. I'll try to come up with better reproducer if I can. > > Thanks, > Eric > > -- BR, Muhammad Usama Anjum
