4.18-stable review patch. If anyone has any objections, please let me know.
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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
Cc: stable@vger.kernel.org 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: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- fs/xattr.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
--- a/fs/xattr.c +++ b/fs/xattr.c @@ -539,7 +539,7 @@ 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) {