On Tue, Oct 18, 2022 at 06:42:04PM -0600, Daniel Xu wrote:
> Hi,
>
> (Going off get_maintainers.pl for fs/namei.c here)
>
> I'm seeing some weird interactions with file capabilities and S_IRUSR
> procfs files. Best I can tell it doesn't occur with real files on my btrfs
> home partition.
>
> Test program:
>
> #include <fcntl.h>
> #include <stdio.h>
>
> int main()
> {
> int fd = open("/proc/self/auxv", O_RDONLY);
> if (fd < 0) {
> perror("open");
> return 1;
> }
>
> printf("ok\n");
> return 0;
> }
>
> Steps to reproduce:
>
> $ gcc main.c
> $ ./a.out
> ok
> $ sudo setcap "cap_net_admin,cap_sys_admin+p" a.out
> $ ./a.out
> open: Permission denied
>
> It's not obvious why this happens, even after spending a few hours
> going through the standard documentation and kernel code. It's
> intuitively odd b/c you'd think adding capabilities to the permitted
> set wouldn't affect functionality.
>
> Best I could tell the -EACCES error occurs in the fallthrough codepath
> inside generic_permission().
>
> Sorry if this is something dumb or obvious.
Hey Daniel,
No, this is neither dumb nor obvious. :)
Basically, if you set fscaps then /proc/self/auxv will be owned by
root:root. You can verify this:
#include <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <stdio.h>
#include <errno.h>
#include <unistd.h>
int main()
{
struct stat st;
printf("%d | %d\n", getuid(), geteuid());
if (stat("/proc/self/auxv", &st)) {
fprintf(stderr, "stat: %d - %m\n", errno);
return 1;
}
printf("stat: %d | %d\n", st.st_uid, st.st_gid);
int fd = open("/proc/self/auxv", O_RDONLY);
if (fd < 0) {
fprintf(stderr, "open: %d - %m\n", errno);
return 1;
}
printf("ok\n");
return 0;
}
$ ./a.out
1000 | 1000
stat: 1000 | 1000
ok
$ sudo setcap "cap_net_admin,cap_sys_admin+p" a.out
$ ./a.out
1000 | 1000
stat: 0 | 0
open: 13 - Permission denied
So acl_permission_check() fails and returns -EACCESS which will cause
generic_permission() to rely on capable_wrt_inode_uidgid() which checks
for CAP_DAC_READ_SEARCH which you don't have as an unprivileged user.
