[PATCH 3.16 28/72] Btrfs: fix use-after-free when using the tree modification log

Ben Hutchings
2019-12-08T13:57:54Z

From: Filipe Manana <fdmanana@suse.com>

commit efad8a853ad2057f96664328a0d327a05ce39c76 upstream.

At ctree.c:get_old_root(), we are accessing a root's header owner field
after we have freed the respective extent buffer. This results in an
use-after-free that can lead to crashes, and when CONFIG_DEBUG_PAGEALLOC
is set, results in a stack trace like the following:

[ 3876.799331] stack segment: 0000 [#1] SMP DEBUG_PAGEALLOC PTI
[ 3876.799363] CPU: 0 PID: 15436 Comm: pool Not tainted 5.3.0-rc3-btrfs-next-54 #1
[ 3876.799385] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.12.0-0-ga698c8995f-prebuilt.qemu.org 04/01/2014
[ 3876.799433] RIP: 0010:btrfs_search_old_slot+0x652/0xd80 [btrfs]
(...)
[ 3876.799502] RSP: 0018:ffff9f08c1a2f9f0 EFLAGS: 00010286
[ 3876.799518] RAX: ffff8dd300000000 RBX: ffff8dd85a7a9348 RCX: 000000038da26000
[ 3876.799538] RDX: 0000000000000000 RSI: ffffe522ce368980 RDI: 0000000000000246
[ 3876.799559] RBP: dae1922adadad000 R08: 0000000008020000 R09: ffffe522c0000000
[ 3876.799579] R10: ffff8dd57fd788c8 R11: 000000007511b030 R12: ffff8dd781ddc000
[ 3876.799599] R13: ffff8dd9e6240578 R14: ffff8dd6896f7a88 R15: ffff8dd688cf90b8
[ 3876.799620] FS: 00007f23ddd97700(0000) GS:ffff8dda20200000(0000) knlGS:0000000000000000
[ 3876.799643] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 3876.799660] CR2: 00007f23d4024000 CR3: 0000000710bb0005 CR4: 00000000003606f0
[ 3876.799682] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[ 3876.799703] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[ 3876.799723] Call Trace:
[ 3876.799735] ? do_raw_spin_unlock+0x49/0xc0
[ 3876.799749] ? _raw_spin_unlock+0x24/0x30
[ 3876.799779] resolve_indirect_refs+0x1eb/0xc80 [btrfs]
[ 3876.799810] find_parent_nodes+0x38d/0x1180 [btrfs]
[ 3876.799841] btrfs_check_shared+0x11a/0x1d0 [btrfs]
[ 3876.799870] ? extent_fiemap+0x598/0x6e0 [btrfs]
[ 3876.799895] extent_fiemap+0x598/0x6e0 [btrfs]
[ 3876.799913] do_vfs_ioctl+0x45a/0x700
[ 3876.799926] ksys_ioctl+0x70/0x80
[ 3876.799938] ? trace_hardirqs_off_thunk+0x1a/0x20
[ 3876.799953] __x64_sys_ioctl+0x16/0x20
[ 3876.799965] do_syscall_64+0x62/0x220
[ 3876.799977] entry_SYSCALL_64_after_hwframe+0x49/0xbe
[ 3876.799993] RIP: 0033:0x7f23e0013dd7
(...)
[ 3876.800056] RSP: 002b:00007f23ddd96ca8 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
[ 3876.800078] RAX: ffffffffffffffda RBX: 00007f23d80210f8 RCX: 00007f23e0013dd7
[ 3876.800099] RDX: 00007f23d80210f8 RSI: 00000000c020660b RDI: 0000000000000003
[ 3876.800626] RBP: 000055fa2a2a2440 R08: 0000000000000000 R09: 00007f23ddd96d7c
[ 3876.801143] R10: 00007f23d8022000 R11: 0000000000000246 R12: 00007f23ddd96d80
[ 3876.801662] R13: 00007f23ddd96d78 R14: 00007f23d80210f0 R15: 00007f23ddd96d80
(...)
[ 3876.805107] ---[ end trace e53161e179ef04f9 ]---

Fix that by saving the root's header owner field into a local variable
before freeing the root's extent buffer, and then use that local variable
when needed.

Fixes: 30b0463a9394d9 ("Btrfs: fix accessing the root pointer in tree mod log functions")
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
fs/btrfs/ctree.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)

--- a/fs/btrfs/ctree.c
+++ b/fs/btrfs/ctree.c
@@ -1411,6 +1411,7 @@ get_old_root(struct btrfs_root *root, u6
struct tree_mod_elem *tm;
struct extent_buffer *eb = NULL;
struct extent_buffer *eb_root;
+ u64 eb_root_owner = 0;
struct extent_buffer *old;
struct tree_mod_root *old_root = NULL;
u64 old_generation = 0;
@@ -1445,6 +1446,7 @@ get_old_root(struct btrfs_root *root, u6
free_extent_buffer(old);
}
} else if (old_root) {
+ eb_root_owner = btrfs_header_owner(eb_root);
btrfs_tree_read_unlock(eb_root);
free_extent_buffer(eb_root);
eb = alloc_dummy_extent_buffer(logical, root->nodesize);
@@ -1462,7 +1464,7 @@ get_old_root(struct btrfs_root *root, u6
if (old_root) {
btrfs_set_header_bytenr(eb, eb->start);
btrfs_set_header_backref_rev(eb, BTRFS_MIXED_BACKREF_REV);
- btrfs_set_header_owner(eb, btrfs_header_owner(eb_root));
+ btrfs_set_header_owner(eb, eb_root_owner);
btrfs_set_header_level(eb, old_root->level);
btrfs_set_header_generation(eb, old_generation);
}