Re: [PATCH 1/1] getxattr: use correct xattr length

[Christian Brauner <christian.brauner@canonical.com>]

On Wed, Jun 13, 2018 at 10:45:37AM -0500, Serge Hallyn wrote:
> On Thu, Jun 07, 2018 at 01:43:48PM +0200, Christian Brauner wrote:
> > 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
> > 
> > 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>
> 
> D'oh, sorry, I thought I had replied to this!

No problem at all. :)

Thanks!
Christian

> 
> Acked-by: Serge Hallyn <serge@hallyn.com>
> 
> thanks,
> Serge
> 
> > ---
> >  fs/xattr.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/fs/xattr.c b/fs/xattr.c
> > index f9cb1db187b7..1bee74682513 100644
> > --- a/fs/xattr.c
> > +++ b/fs/xattr.c
> > @@ -539,7 +539,7 @@ getxattr(struct dentry *d, const char __user *name, void __user *value,
> >  	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) {
> > -- 
> > 2.17.0