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SubjectRe: [PATCH 1/1] getxattr: use correct xattr length
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!

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

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