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Subject[GIT PULL] capabilities: fix buffer overread on very short xattr
Hi Linus,

Please pull this capabilities fix for v4.15.

The following changes since commit 30a7acd573899fd8b8ac39236eff6468b195ac7d:

Linux 4.15-rc6 (2017-12-31 14:47:43 -0800)

are available in the git repository at:

git:// for-linus

for you to fetch changes up to dc32b5c3e6e2ef29cef76d9ce1b92d394446150e:

capabilities: fix buffer overread on very short xattr (2018-01-02 20:49:13 +1100)

Eric Biggers (1):
capabilities: fix buffer overread on very short xattr

security/commoncap.c | 21 +++++++++------------
1 file changed, 9 insertions(+), 12 deletions(-)


commit dc32b5c3e6e2ef29cef76d9ce1b92d394446150e
Author: Eric Biggers <>
Date: Mon Jan 1 09:28:31 2018 -0600

capabilities: fix buffer overread on very short xattr

If userspace attempted to set a "security.capability" xattr shorter than
4 bytes (e.g. 'setfattr -n security.capability -v x file'), then
cap_convert_nscap() read past the end of the buffer containing the xattr
value because it accessed the ->magic_etc field without verifying that
the xattr value is long enough to contain that field.

Fix it by validating the xattr value size first.

This bug was found using syzkaller with KASAN. The KASAN report was as
follows (cleaned up slightly):

BUG: KASAN: slab-out-of-bounds in cap_convert_nscap+0x514/0x630 security/commoncap.c:498
Read of size 4 at addr ffff88002d8741c0 by task syz-executor1/2852

CPU: 0 PID: 2852 Comm: syz-executor1 Not tainted 4.15.0-rc6-00200-gcc0aac99d977 #253
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.11.0-20171110_100015-anatol 04/01/2014
Call Trace:
__dump_stack lib/dump_stack.c:17 [inline]
dump_stack+0xe3/0x195 lib/dump_stack.c:53
print_address_description+0x73/0x260 mm/kasan/report.c:252
kasan_report_error mm/kasan/report.c:351 [inline]
kasan_report+0x235/0x350 mm/kasan/report.c:409
cap_convert_nscap+0x514/0x630 security/commoncap.c:498
setxattr+0x2bd/0x350 fs/xattr.c:446
path_setxattr+0x168/0x1b0 fs/xattr.c:472
SYSC_setxattr fs/xattr.c:487 [inline]
SyS_setxattr+0x36/0x50 fs/xattr.c:483

Fixes: 8db6c34f1dbc ("Introduce v3 namespaced file capabilities")
Cc: <> # v4.14+
Signed-off-by: Eric Biggers <>
Reviewed-by: Serge Hallyn <>
Signed-off-by: James Morris <>

diff --git a/security/commoncap.c b/security/commoncap.c
index 4f8e093..48620c9 100644
--- a/security/commoncap.c
+++ b/security/commoncap.c
@@ -348,21 +348,18 @@ static __u32 sansflags(__u32 m)

-static bool is_v2header(size_t size, __le32 magic)
+static bool is_v2header(size_t size, const struct vfs_cap_data *cap)
- __u32 m = le32_to_cpu(magic);
if (size != XATTR_CAPS_SZ_2)
return false;
- return sansflags(m) == VFS_CAP_REVISION_2;
+ return sansflags(le32_to_cpu(cap->magic_etc)) == VFS_CAP_REVISION_2;

-static bool is_v3header(size_t size, __le32 magic)
+static bool is_v3header(size_t size, const struct vfs_cap_data *cap)
- __u32 m = le32_to_cpu(magic);
if (size != XATTR_CAPS_SZ_3)
return false;
- return sansflags(m) == VFS_CAP_REVISION_3;
+ return sansflags(le32_to_cpu(cap->magic_etc)) == VFS_CAP_REVISION_3;

@@ -405,7 +402,7 @@ int cap_inode_getsecurity(struct inode *inode, const char *name, void **buffer,

fs_ns = inode->i_sb->s_user_ns;
cap = (struct vfs_cap_data *) tmpbuf;
- if (is_v2header((size_t) ret, cap->magic_etc)) {
+ if (is_v2header((size_t) ret, cap)) {
/* If this is sizeof(vfs_cap_data) then we're ok with the
* on-disk value, so return that. */
if (alloc)
@@ -413,7 +410,7 @@ int cap_inode_getsecurity(struct inode *inode, const char *name, void **buffer,
return ret;
- } else if (!is_v3header((size_t) ret, cap->magic_etc)) {
+ } else if (!is_v3header((size_t) ret, cap)) {
return -EINVAL;
@@ -470,9 +467,9 @@ static kuid_t rootid_from_xattr(const void *value, size_t size,
return make_kuid(task_ns, rootid);

-static bool validheader(size_t size, __le32 magic)
+static bool validheader(size_t size, const struct vfs_cap_data *cap)
- return is_v2header(size, magic) || is_v3header(size, magic);
+ return is_v2header(size, cap) || is_v3header(size, cap);

@@ -495,7 +492,7 @@ int cap_convert_nscap(struct dentry *dentry, void **ivalue, size_t size)

if (!*ivalue)
return -EINVAL;
- if (!validheader(size, cap->magic_etc))
+ if (!validheader(size, cap))
return -EINVAL;
if (!capable_wrt_inode_uidgid(inode, CAP_SETFCAP))
return -EPERM;
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