[GIT PULL] capabilities: fix buffer overread on very short xattr

[James Morris]

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://git.kernel.org/pub/scm/linux/kernel/git/jmorris/linux-security.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 <ebiggers at google.com>
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
         entry_SYSCALL_64_fastpath+0x18/0x85
    
    Fixes: 8db6c34f1dbc ("Introduce v3 namespaced file capabilities")
    Cc: <stable at vger.kernel.org> # v4.14+
    Signed-off-by: Eric Biggers <ebiggers at google.com>
    Reviewed-by: Serge Hallyn <serge at hallyn.com>
    Signed-off-by: James Morris <james.l.morris at oracle.com>

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)
 	return m & ~VFS_CAP_FLAGS_EFFECTIVE;
 }
 
-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,
 		else
 			kfree(tmpbuf);
 		return ret;
-	} else if (!is_v3header((size_t) ret, cap->magic_etc)) {
+	} else if (!is_v3header((size_t) ret, cap)) {
 		kfree(tmpbuf);
 		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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