[PATCH 1/3] xfs: don't overflow xattr listent buffer

Darrick J. Wong
2019-02-13T20:50:51Z

From: Darrick J. Wong <darrick.wong@oracle.com>

For VFS listxattr calls, xfs_xattr_put_listent calls
__xfs_xattr_put_listent twice if it sees an attribute
"trusted.SGI_ACL_FILE": once for that name, and again for
"system.posix_acl_access". Unfortunately, if we happen to run out of
buffer space while emitting the first name, we set count to -1 (so that
we can feed ERANGE to the caller). The second invocation doesn't check that
the context parameters make sense and overwrites the byte before the
buffer, triggering a KASAN report:

==================================================================
BUG: KASAN: slab-out-of-bounds in strncpy+0xb3/0xd0
Write of size 1 at addr ffff88807fbd317f by task syz/1113

CPU: 3 PID: 1113 Comm: syz Not tainted 5.0.0-rc6-xfsx #rc6
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.10.2-1ubuntu1 04/01/2014
Call Trace:
dump_stack+0xcc/0x180
print_address_description+0x6c/0x23c
kasan_report.cold.3+0x1c/0x35
strncpy+0xb3/0xd0
__xfs_xattr_put_listent+0x1a9/0x2c0 [xfs]
xfs_attr_list_int_ilocked+0x11af/0x1800 [xfs]
xfs_attr_list_int+0x20c/0x2e0 [xfs]
xfs_vn_listxattr+0x225/0x320 [xfs]
listxattr+0x11f/0x1b0
path_listxattr+0xbd/0x130
do_syscall_64+0x139/0x560

While we're at it we add an assert to the other put_listent to avoid
this sort of thing ever happening to the attrlist_by_handle code.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
---
fs/xfs/xfs_attr_list.c | 1 +
fs/xfs/xfs_xattr.c | 3 +++
2 files changed, 4 insertions(+)

diff --git a/fs/xfs/xfs_attr_list.c b/fs/xfs/xfs_attr_list.c
index a58034049995..3d213a7394c5 100644
--- a/fs/xfs/xfs_attr_list.c
+++ b/fs/xfs/xfs_attr_list.c
@@ -555,6 +555,7 @@ xfs_attr_put_listent(
attrlist_ent_t *aep;
int arraytop;

+ ASSERT(!context->seen_enough);
ASSERT(!(context->flags & ATTR_KERNOVAL));
ASSERT(context->count >= 0);
ASSERT(context->count < (ATTR_MAX_VALUELEN/8));
diff --git a/fs/xfs/xfs_xattr.c b/fs/xfs/xfs_xattr.c
index 63ee1d5bf1d7..9a63016009a1 100644
--- a/fs/xfs/xfs_xattr.c
+++ b/fs/xfs/xfs_xattr.c
@@ -129,6 +129,9 @@ __xfs_xattr_put_listent(
char *offset;
int arraytop;

+ if (context->count < 0 || context->seen_enough)
+ return;
+
if (!context->alist)
goto compute_size;