linux-kernel - Odd interaction with file capabilities and procfs files

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [thread-next>] [day] [month] [year] [list]

Message-Id: <f1e63e54-d88d-4b69-86f1-c0b4a0fd8035@app.fastmail.com>
Date:   Tue, 18 Oct 2022 18:42:04 -0600
From:   "Daniel Xu" <dxu@...uu.xyz>
To:     viro@...iv.linux.org.uk, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Odd interaction with file capabilities and procfs files

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.

Thanks,
Daniel