Bernd Edlinger <[email protected]> writes: > On 11/21/25 10:35, Bernd Edlinger wrote: >> On 11/21/25 08:18, Eric W. Biederman wrote: >>> Bernd Edlinger <[email protected]> writes: >>> >>>> Hi Eric, >>>> >>>> thanks for you valuable input on the topic. >>>> >>>> On 11/21/25 00:50, Eric W. Biederman wrote: >>>>> "Eric W. Biederman" <[email protected]> writes: >>>>> >>>>>> Instead of computing the new cred before we pass the point of no >>>>>> return compute the new cred just before we use it. >>>>>> >>>>>> This allows the removal of fs_struct->in_exec and cred_guard_mutex. >>>>>> >>>>>> I am not certain why we wanted to compute the cred for the new >>>>>> executable so early. Perhaps I missed something but I did not see any >>>>>> common errors being signaled. So I don't think we loose anything by >>>>>> computing the new cred later. >>>>> >>>>> I should add that the permission checks happen in open_exec, >>>>> everything that follows credential wise is just about representing in >>>>> struct cred the credentials the new executable will have. >>>>> >>>>> So I am really at a loss why we have had this complicated way of >>>>> computing of computed the credentials all of these years full of >>>>> time of check to time of use problems. >>>>> >>>> >>>> Well, I think I see a problem with your patch: >>>> >>>> When the security engine gets the LSM_UNSAFE_PTRACE flag, it might >>>> e.g. return -EPERM in bprm_creds_for_exec in the apparmor, selinux >>>> or the smack security engines at least. Previously that callback >>>> was called before the point of no return, and the return code should >>>> be returned as a return code the the caller of execve. But if we move >>>> that check after the point of no return, the caller will get killed >>>> due to the failed security check. >>>> >>>> Or did I miss something? >>> >>> I think we definitely need to document this change in behavior. I would >>> call ending the exec with SIGSEGV vs -EPERM a quality of implementation >>> issue. The exec is failing one way or the other so I don't see it as a >>> correctness issue. >>> >>> In the case of ptrace in general I think it is a bug if the mere act of >>> debugging a program changes it's behavior. So which buggy behavior >>> should we prefer? SIGSEGV where it is totally clear that the behavior >>> has changed or -EPERM and ask the debugged program to handle it. >>> I lean towards SIGSEGV because then it is clear the code should not >>> handle it. >>> >>> In the case of LSM_UNSAFE_NO_NEW_PRIVS I believe the preferred way to >>> handle unexpected things happening is to terminate the application. >>> >>> In the case of LSM_UNSAFE_SHARE -EPERM might be better. I don't know >>> of any good uses of any good uses of sys_clone(CLONE_FS ...) outside >>> of CLONE_THREAD. >>> >>> >>> Plus all of these things are only considerations if we are exec'ing a >>> program that transitions to a different set of credentials. Something >>> that happens but is quite rare itself. >>> >>> In practice I don't expect there is anything that depends on the exact >>> behavior of what happens when exec'ing a suid executable to gain >>> privileges when ptraced. The closes I can imagine is upstart and >>> I think upstart ran as root when ptracing other programs so there is no >>> gaining of privilege and thus no reason for a security module to >>> complain. >>> >>> Who knows I could be wrong, and someone could actually care. Which is >>> hy I think we should document it.>> >> >> >> Well, I dont know for sure, but the security engine could deny the execution >> for any reason, not only because of being ptraced. >> Maybe there can be a policy which denies user X to execute e.g. any suid >> programs. >> >> >> Bernd. >> > > Hmm, funny.. > > I installed this patch on top of > > commit fd95357fd8c6778ac7dea6c57a19b8b182b6e91f (HEAD -> master, > origin/master, origin/HEAD) > Merge: c966813ea120 7b6216baae75 > Author: Linus Torvalds <[email protected]> > Date: Thu Nov 20 11:04:37 2025 -0800 > > but it does panic when I try to boot: > > [ 0.870539] TERM=1inux > [ 0.870573] Starting init: /bin/sh exists but couldn't execute it (error > -14) 0.8705751 Kernel panic- not syncing: No working init found. Try passing > i mit= option to kernel. See Linux Documentation/admin-guide/init.rst for > guidance > [ 0.870577] CPU: UID: 0 PID: 1 Comm: sh Not tainted 6.18.0-rc6+ #1 > PREEMPT(voluntary) > [ 0.870579] Hardware name: innotek GmbH VirtualBox/VirtualBox, BIOS > VirtualBo x 12/01/2006 > [ 0.870580] Call Trace: > [ 0.870590] <TASK> > [ 0.870592] vpanic+0x36d/0x380 > [ 0.870607] ? __pfx_kernel_init+0x10/0x10 > [ 0.870615] panic+0x5b/0x60 > [ 0.870617] kernel_init+0x17d/0x1c0 > [ 0.870623] ret_from_fork+0x124/0x150 > [ 0.870625} ? __pfx_kernel_init+0x10/0x10 > [ 0.870627] ret_from_fork_asm+0x1a/0x30 > [ 0.870632] </TASK> > [ 0.8706631 Kernel Offset: 0x3a800000 from Oxffffffff81000000 (relocation > ran ge: 0xffffffff80000000-0xffffffffbfffffff) > [ 0.880034] ---[ end Kernel panic - not syncing: No working init found. Try > passing init option to kernel. See Linux Documentation/admin-guide/init.rst > for guidance. 1---` > > > Is that a known problem?
Nope. It looks like the code needs a little bit bug fixing testing. I will take see about taking a look. Eric
