[PATCH 3.16 157/328] getxattr: use correct xattr length

Ben Hutchings
2018-12-09T22:12:23Z

From: Christian Brauner <christian@brauner.io>

commit 82c9a927bc5df6e06b72d206d24a9d10cced4eb5 upstream.

When running in a container with a user namespace, if you call getxattr
with name = "system.posix_acl_access" and size % 8 != 4, then getxattr
silently skips the user namespace fixup that it normally does resulting in
un-fixed-up data being returned.
This is caused by posix_acl_fix_xattr_to_user() being passed the total
buffer size and not the actual size of the xattr as returned by
vfs_getxattr().
This commit passes the actual length of the xattr as returned by
vfs_getxattr() down.

A reproducer for the issue is:

touch acl_posix

setfacl -m user:0:rwx acl_posix

and the compile:

#define _GNU_SOURCE
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <unistd.h>
#include <attr/xattr.h>

/* Run in user namespace with nsuid 0 mapped to uid != 0 on the host. */
int main(int argc, void **argv)
{
ssize_t ret1, ret2;
char buf1[128], buf2[132];
int fret = EXIT_SUCCESS;
char *file;

if (argc < 2) {
fprintf(stderr,
"Please specify a file with "
"\"system.posix_acl_access\" permissions set\n");
_exit(EXIT_FAILURE);
}
file = argv[1];

ret1 = getxattr(file, "system.posix_acl_access",
buf1, sizeof(buf1));
if (ret1 < 0) {
fprintf(stderr, "%s - Failed to retrieve "
"\"system.posix_acl_access\" "
"from \"%s\"\n", strerror(errno), file);
_exit(EXIT_FAILURE);
}

ret2 = getxattr(file, "system.posix_acl_access",
buf2, sizeof(buf2));
if (ret2 < 0) {
fprintf(stderr, "%s - Failed to retrieve "
"\"system.posix_acl_access\" "
"from \"%s\"\n", strerror(errno), file);
_exit(EXIT_FAILURE);
}

if (ret1 != ret2) {
fprintf(stderr, "The value of \"system.posix_acl_"
"access\" for file \"%s\" changed "
"between two successive calls\n", file);
_exit(EXIT_FAILURE);
}

for (ssize_t i = 0; i < ret2; i++) {
if (buf1[i] == buf2[i])
continue;

fprintf(stderr,
"Unexpected different in byte %zd: "
"%02x != %02x\n", i, buf1[i], buf2[i]);
fret = EXIT_FAILURE;
}

if (fret == EXIT_SUCCESS)
fprintf(stderr, "Test passed\n");
else
fprintf(stderr, "Test failed\n");

_exit(fret);
}
and run:

./tester acl_posix

On a non-fixed up kernel this should return something like:

root@c1:/# ./t
Unexpected different in byte 16: ffffffa0 != 00
Unexpected different in byte 17: ffffff86 != 00
Unexpected different in byte 18: 01 != 00

and on a fixed kernel:

root@c1:~# ./t
Test passed

Fixes: 2f6f0654ab61 ("userns: Convert vfs posix_acl support to use kuids and kgids")
Link: https://bugzilla.kernel.org/show_bug.cgi?id=199945
Reported-by: Colin Watson <cjwatson@ubuntu.com>
Signed-off-by: Christian Brauner <christian@brauner.io>
Acked-by: Serge Hallyn <serge@hallyn.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
fs/xattr.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

--- a/fs/xattr.c
+++ b/fs/xattr.c
@@ -466,7 +466,7 @@ getxattr(struct dentry *d, const char __
if (error > 0) {
if ((strcmp(kname, XATTR_NAME_POSIX_ACL_ACCESS) == 0) ||
(strcmp(kname, XATTR_NAME_POSIX_ACL_DEFAULT) == 0))
- posix_acl_fix_xattr_to_user(kvalue, size);
+ posix_acl_fix_xattr_to_user(kvalue, error);
if (size && copy_to_user(value, kvalue, error))
error = -EFAULT;
} else if (error == -ERANGE && size >= XATTR_SIZE_MAX) {