Re: [PATCH v5 5/6] prctl: Allow checkpoint/restore capable processes to change exe link
From: Christian Brauner <hidden>
Date: 2020-07-15 15:20:40
Also in:
linux-fsdevel, lkml, selinux
On Wed, Jul 15, 2020 at 04:49:53PM +0200, Adrian Reber wrote:
From: Nicolas Viennot <redacted> Allow CAP_CHECKPOINT_RESTORE capable users to change /proc/self/exe. This commit also changes the permission error code from -EINVAL to -EPERM for consistency with the rest of the prctl() syscall when checking capabilities.
I agree that EINVAL seems weird here but this is a potentially user visible change. Might be nice to have the EINVAL->EPERM change be an additional patch on top after this one so we can revert it in case it breaks someone (unlikely though). I can split this out myself though so no need to resend for that alone. What I would also prefer is to have some history in the commit message tbh. The reason is that when we started discussing that specific change I had to hunt down the history of changing /proc/self/exe and had to dig up and read through ancient threads on lore to come up with the explanation why this is placed under a capability. The commit message should then also mention that there are other ways to change the /proc/self/exe link that don't require capabilities and that /proc/self/exe itself is not something userspace should rely on for security. Mainly so that in a few months/years we can read through that commit message and go "Weird, but ok.". :) But maybe I can just rewrite this myself so you don't have to go through the trouble. This is really not pedantry it's just that it's a lot of work digging up the reasons for a piece of code existing when it's really not obvious. :) Christian
quoted hunk ↗ jump to hunk
Signed-off-by: Nicolas Viennot <redacted> Signed-off-by: Adrian Reber <redacted> --- kernel/sys.c | 12 +++++++----- 1 file changed, 7 insertions(+), 5 deletions(-)diff --git a/kernel/sys.c b/kernel/sys.c index 00a96746e28a..dd59b9142b1d 100644 --- a/kernel/sys.c +++ b/kernel/sys.c@@ -2007,12 +2007,14 @@ static int prctl_set_mm_map(int opt, const void __user *addr, unsigned long data if (prctl_map.exe_fd != (u32)-1) { /* - * Make sure the caller has the rights to - * change /proc/pid/exe link: only local sys admin should - * be allowed to. + * Check if the current user is checkpoint/restore capable. + * At the time of this writing, it checks for CAP_SYS_ADMIN + * or CAP_CHECKPOINT_RESTORE. + * Note that a user with access to ptrace can masquerade an + * arbitrary program as any executable, even setuid ones. */ - if (!ns_capable(current_user_ns(), CAP_SYS_ADMIN)) - return -EINVAL; + if (!checkpoint_restore_ns_capable(current_user_ns())) + return -EPERM; error = prctl_set_mm_exe_file(mm, prctl_map.exe_fd); if (error)-- 2.26.2