Re: [PATCH v2] ext4: Fix possible use-after-free in ext4_find_extent
From: Tudor Ambarus
Date: Fri Dec 30 2022 - 06:42:56 EST
Hi, Eric,
On 30.12.2022 08:56, Eric Biggers wrote:
On Fri, Dec 30, 2022 at 08:29:31AM +0200, Tudor Ambarus wrote:
syzbot reported a use-after-free Read in ext4_find_extent that is hit when
using a corrupted file system. The bug was reported on Android 5.15, but
using the same reproducer triggers the bug on v6.2-rc1 as well.
Fix the use-after-free by checking the extent header magic. An alternative
would be to check the values of EXT4_{FIRST,LAST}_{EXTENT,INDEX} used in
ext4_ext_binsearch() and ext4_ext_binsearch_idx(), so that we make sure
that pointers returned by EXT4_{FIRST,LAST}_{EXTENT,INDEX} don't exceed the
bounds of the extent tree node. But this alternative will not squash
the bug for the cases where eh->eh_entries fit into eh->eh_max. We could
also try to check the sanity of the path, but costs more than checking just
the header magic, so stick to the header magic sanity check.
Link: https://syzkaller.appspot.com/bug?id=be6e90ce70987950e6deb3bac8418344ca8b96cd
Reported-by: syzbot+0827b4b52b5ebf65f219@xxxxxxxxxxxxxxxxxxxxxxxxx
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Tudor Ambarus <tudor.ambarus@xxxxxxxxxx>
---
v2: drop wrong/uneeded le16_to_cpu() conversion for eh->eh_magic
fs/ext4/extents.c | 6 ++++++
1 file changed, 6 insertions(+)
diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
index 9de1c9d1a13d..bedc8c098449 100644
--- a/fs/ext4/extents.c
+++ b/fs/ext4/extents.c
@@ -894,6 +894,12 @@ ext4_find_extent(struct inode *inode, ext4_lblk_t block,
gfp_flags |= __GFP_NOFAIL;
eh = ext_inode_hdr(inode);
+ if (eh->eh_magic != EXT4_EXT_MAGIC) {
+ EXT4_ERROR_INODE(inode, "Extent header has invalid magic.");
+ ret = -EFSCORRUPTED;
+ goto err;
+ }
+
This is (incompletely) validating the extent header in the inode. Isn't that
supposed to happen when the inode is loaded? See how __ext4_iget() calls
ext4_ext_check_inode(). Why isn't that working here?
Seems that __ext4_iget() is not called on writes. You can find below the
sequence of calls that leads to the bug. The debug was done on v6.2-rc1.
I assume the extents check is no longer done on writes since
commit 7a262f7c69163cd4811f2f838faef5c5b18439c9. The commit doesn't
specify the reason though.
Cheers,
ta
---
#20 0xffffffff85000087 in entry_SYSCALL_64 ()
at /home/atudor/work/aosp/arch/x86/entry/entry_64.S:120
#19 do_syscall_64 (regs=0xffffc90001fbff58, nr=82542)
at /home/atudor/work/aosp/arch/x86/entry/common.c:80
#18 0xffffffff84f27caf in do_syscall_x64 (regs=0xffffc90001fbff58, nr=82542)
at /home/atudor/work/aosp/arch/x86/entry/common.c:50
#17 __x64_sys_write (regs=<optimized out>) at
/home/atudor/work/aosp/fs/read_write.c:646
#16 __se_sys_write (fd=4, buf=536871424, count=0)
at /home/atudor/work/aosp/fs/read_write.c:646
#15 0xffffffff81c668fb in __do_sys_write (fd=4, buf=0x20000200
"threaded", count=0)
at /home/atudor/work/aosp/fs/read_write.c:649
#14 0xffffffff81c66758 in ksys_write (fd=fd@entry=4,
buf=buf@entry=0x20000200 "threaded",
count=392007683) at /home/atudor/work/aosp/fs/read_write.c:637
#13 vfs_write (file=file@entry=0xffff8881352f9900, buf=<optimized out>,
buf@entry=0x20000200 "threaded", count=<optimized out>,
count@entry=392007683,
pos=<optimized out>, pos@entry=0xffffc90001fbfe80)
at /home/atudor/work/aosp/fs/read_write.c:584
#12 new_sync_write (filp=<optimized out>, buf=0x20000200 "threaded",
len=392007683,
ppos=0xffffc90001fbfe80) at /home/atudor/work/aosp/fs/read_write.c:491
#11 0xffffffff81c65ba5 in call_write_iter (file=0xffff8881352f9900,
kio=<optimized out>,
iter=<optimized out>) at /home/atudor/work/aosp/include/linux/fs.h:2186
#10 0xffffffff81f11e78 in ext4_file_write_iter (iocb=<optimized out>,
from=0xffffc90001fbfd70) at /home/atudor/work/aosp/fs/ext4/file.c:700
#9 0xffffffff81f148ff in ext4_buffered_write_iter (iocb=0xffffc90001fbfd20,
from=from@entry=0xffffc90001fbfd70) at
/home/atudor/work/aosp/fs/ext4/file.c:285
#8 0xffffffff81a4aef4 in generic_perform_write
(iocb=iocb@entry=0xffffc90001fbfd20,
i=<optimized out>, i@entry=0xffffc90001fbfd70)
at /home/atudor/work/aosp/mm/filemap.c:3772
#7 0xffffffff81f682b9 in ext4_da_write_begin (file=<optimized out>,
mapping=<optimized out>, pos=<optimized out>, len=<optimized out>,
pagep=<optimized out>, fsdata=<optimized out>)
at /home/atudor/work/aosp/fs/ext4/inode.c:3082
#6 0xffffffff81f66995 in ext4_block_write_begin
(page=page@entry=0xffffea0004859500,
pos=pos@entry=32768, len=len@entry=4096,
get_block=0xffffffff81f47b10 <ext4_da_get_block_prep>)
at /home/atudor/work/aosp/fs/ext4/inode.c:1098
#5 ext4_da_get_block_prep (inode=0xffff888117bbe7a8, iblock=16,
bh=0xffff888117b5db28,
create=<optimized out>) at /home/atudor/work/aosp/fs/ext4/inode.c:1870
#4 ext4_da_map_blocks (inode=0xffff888117bbe7a8, iblock=16,
bh=0xffff888117b5db28,
map=<optimized out>) at /home/atudor/work/aosp/fs/ext4/inode.c:1806
#3 0xffffffff81f48526 in ext4_insert_delayed_block
(inode=0xffff888117bbe7a8,
lblk=<optimized out>) at /home/atudor/work/aosp/fs/ext4/inode.c:1696
#2 0xffffffff81ef9b96 in ext4_clu_mapped
(inode=inode@entry=0xffff888117bbe7a8, lclu=1)
at /home/atudor/work/aosp/fs/ext4/extents.c:5809
#1 ext4_find_extent (inode=inode@entry=0xffff888117bbe7a8, block=16,
orig_path=orig_path@entry=0x0 <fixed_percpu_data>, flags=flags@entry=0)
at /home/atudor/work/aosp/fs/ext4/extents.c:931
#0 ext4_ext_binsearch_idx (path=0xffff888136ad3800, block=16,
inode=<optimized out>)
at /home/atudor/work/aosp/fs/ext4/extents.c:768