[PATCH] ext4: fix bug in extents parsing when number of entries in header is zero

Luís Henriques
2022-08-05T14:00:55Z

When walking through an inode extents, the ext4_ext_binsearch_idx() function
assumes that the extent header has been previously validated. However,
there are no checks that verify that the number of entries (eh->eh_entries)
is non-zero. And this will lead to problems because the EXT_FIRST_INDEX()
and EXT_LAST_INDEX() will return garbage and result in this:

[ 135.245946] ------------[ cut here ]------------
[ 135.247579] kernel BUG at fs/ext4/extents.c:2258!
[ 135.249045] invalid opcode: 0000 [#1] PREEMPT SMP
[ 135.250320] CPU: 2 PID: 238 Comm: tmp118 Not tainted 5.19.0-rc8+ #4
[ 135.252067] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.15.0-0-g2dd4b9b-rebuilt.opensuse.org 04/01/2014
[ 135.255065] RIP: 0010:ext4_ext_map_blocks+0xc20/0xcb0
[ 135.256475] Code:
[ 135.261433] RSP: 0018:ffffc900005939f8 EFLAGS: 00010246
[ 135.262847] RAX: 0000000000000024 RBX: ffffc90000593b70 RCX: 0000000000000023
[ 135.264765] RDX: ffff8880038e5f10 RSI: 0000000000000003 RDI: ffff8880046e922c
[ 135.266670] RBP: ffff8880046e9348 R08: 0000000000000001 R09: ffff888002ca580c
[ 135.268576] R10: 0000000000002602 R11: 0000000000000000 R12: 0000000000000024
[ 135.270477] R13: 0000000000000000 R14: 0000000000000024 R15: 0000000000000000
[ 135.272394] FS: 00007fdabdc56740(0000) GS:ffff88807dd00000(0000) knlGS:0000000000000000
[ 135.274510] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 135.276075] CR2: 00007ffc26bd4f00 CR3: 0000000006261004 CR4: 0000000000170ea0
[ 135.277952] Call Trace:
[ 135.278635] <TASK>
[ 135.279247] ? preempt_count_add+0x6d/0xa0
[ 135.280358] ? percpu_counter_add_batch+0x55/0xb0
[ 135.281612] ? _raw_read_unlock+0x18/0x30
[ 135.282704] ext4_map_blocks+0x294/0x5a0
[ 135.283745] ? xa_load+0x6f/0xa0
[ 135.284562] ext4_mpage_readpages+0x3d6/0x770
[ 135.285646] read_pages+0x67/0x1d0
[ 135.286492] ? folio_add_lru+0x51/0x80
[ 135.287441] page_cache_ra_unbounded+0x124/0x170
[ 135.288510] filemap_get_pages+0x23d/0x5a0
[ 135.289457] ? path_openat+0xa72/0xdd0
[ 135.290332] filemap_read+0xbf/0x300
[ 135.291158] ? _raw_spin_lock_irqsave+0x17/0x40
[ 135.292192] new_sync_read+0x103/0x170
[ 135.293014] vfs_read+0x15d/0x180
[ 135.293745] ksys_read+0xa1/0xe0
[ 135.294461] do_syscall_64+0x3c/0x80
[ 135.295284] entry_SYSCALL_64_after_hwframe+0x46/0xb0

Unfortunately, __ext4_ext_check() only verifies that eh->eh_entries doesn't
exceed eh->eh_max. And since an empty leaf seems to be a valid value in
same cases, adding this extra check there isn't an option.

This patch simply adds the check directly in ext4_ext_binsearch_idx() and
propagates this error so that the kernel doesn't hit this BUG_ON() in
ext4_ext_determine_hole().

Link: https://bugzilla.kernel.org/show_bug.cgi?id=215941
Signed-off-by: Luís Henriques <lhenriques@suse.de>
---
fs/ext4/extents.c | 13 ++++++++++---
1 file changed, 10 insertions(+), 3 deletions(-)

Hi!

This bug is easily reproducible using the filesystem image provided --
it's just a matter of mounting it and run:

$ cat /mnt/foo/bar/xattr

Anyway, I hope my analysis of the bug is correct -- the root cause seems
to be an extent header with an invalid value for in eh_entries, which will
later cause the BUG_ON().

Cheers,
--
Luís

diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
index c148bb97b527..53cfe2c681c4 100644
--- a/fs/ext4/extents.c
+++ b/fs/ext4/extents.c
@@ -738,7 +738,7 @@ void ext4_ext_drop_refs(struct ext4_ext_path *path)
* binary search for the closest index of the given block
* the header must be checked before calling this
*/
-static void
+static int
ext4_ext_binsearch_idx(struct inode *inode,
struct ext4_ext_path *path, ext4_lblk_t block)
{
@@ -748,6 +748,11 @@ ext4_ext_binsearch_idx(struct inode *inode,

ext_debug(inode, "binsearch for %u(idx): ", block);

+ if (eh->eh_entries == 0) {
+ EXT4_ERROR_INODE(inode, "No entries in extent header!");
+ return -EFSCORRUPTED;
+ }
+
l = EXT_FIRST_INDEX(eh) + 1;
r = EXT_LAST_INDEX(eh);
while (l <= r) {
@@ -791,7 +796,7 @@ ext4_ext_binsearch_idx(struct inode *inode,
BUG_ON(chix != path->p_idx);
}
#endif
-
+ return 0;
}

/*
@@ -919,7 +924,9 @@ ext4_find_extent(struct inode *inode, ext4_lblk_t block,
ext_debug(inode, "depth %d: num %d, max %d\n",
ppos, le16_to_cpu(eh->eh_entries), le16_to_cpu(eh->eh_max));

- ext4_ext_binsearch_idx(inode, path + ppos, block);
+ ret = ext4_ext_binsearch_idx(inode, path + ppos, block);
+ if (ret < 0)
+ goto err;
path[ppos].p_block = ext4_idx_pblock(path[ppos].p_idx);
path[ppos].p_depth = i;
path[ppos].p_ext = NULL;