From mboxrd@z Thu Jan 1 00:00:00 1970 From: Vasiliy Kulikov Subject: Re: Documenting execve() and EAGAIN Date: Mon, 26 May 2014 22:11:20 +0400 Message-ID: <20140526181120.GB11145@cachalot> References: <537CEC90.7060000@gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: QUOTED-PRINTABLE Return-path: Content-Disposition: inline In-Reply-To: <537CEC90.7060000-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> Sender: linux-man-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: "Michael Kerrisk (man-pages)" Cc: KOSAKI Motohiro , "linux-man-u79uwXL29TY76Z2rM5mHXA@public.gmane.org" , lkml List-Id: linux-man@vger.kernel.org Hi Michael, On Wed, May 21, 2014 at 20:12 +0200, Michael Kerrisk (man-pages) wrote: > Vasily (and Motohiro), >=20 > Sometime ago, Motohiro raised a documentation bug > ( https://bugzilla.kernel.org/show_bug.cgi?id=3D42704 ) which=20 > relates to your commit 72fa59970f8698023045ab0713d66f3f4f96945c > ("move RLIMIT_NPROC check from set_user() to do_execve_common()") >=20 > I have attempted to document this, and I would like to ask you > (and Motohiro) if you would review the text proposed below for > the exceve(2) man page. >=20 > Thank you, >=20 > Michael >=20 >=20 > ERRORS > EAGAIN (since Linux 3.1) > Having changed its real UID using one of the set*uid() > calls, the caller was=E2=80=94and is now still=E2= =80=94above its > RLIMIT_NPROC resource limit (see setrlimit(2)). For a > more detailed explanation of this error, see NOTES. >=20 > NOTES > execve() and EAGAIN > A more detailed explanation of the EAGAIN error that can occur > (since Linux 3.1) when calling execve() is as follows. >=20 > The EAGAIN error can occur when a preceding call to setuid(2), > setreuid(2), or setresuid(2) caused the real user ID of the > process to change, and that change caused the process to > exceed its RLIMIT_NPROC resource limit (i.e., the number of > processes belonging to the new real UID exceeds the resource > limit). In Linux 3.0 and earlier, this caused the set*uid() > call to fail. >=20 > Since Linux 3.1, the scenario just described no longer causes > the set*uid() call to fail, because it too often led to secu=E2= =80=90 > rity holes because buggy applications didn't check the return > status and assumed that=E2=80=94if the caller had root privil= eges=E2=80=94the > call would always succeed. Instead, the set*uid() calls now > successfully change real UID, but the kernel sets an internal > flag, named PF_NPROC_EXCEEDED, to note that the RLIMIT_NPROC > resource limit has been exceeded. If the resource limit is > still exceeded at the time of a subsequent execve() call, that > call fails with the error EAGAIN. This kernel logic ensures > that the RLIMIT_NPROC resource limit is still enforced for the > common privileged daemon workflow=E2=80=94namely, fork(2)+ se= t*uid()+ > execve(2). >=20 > If the resource limit was not still exceeded at the time of > the execve() call (because other processes belonging to this > real UID terminated between the set*uid() call and the > execve() call), then the execve() call succeeds and the kernel > clears the PF_NPROC_EXCEEDED process flag. The flag is also > cleared if a subsequent call to fork(2) by this process suc=E2= =80=90 > ceeds. Probably explicitly state that NPROC check on execve() is processed onl= y in case of a previous set*uid() call? If there was no previous set*uid() call the semantics of execve() checks are the same as before (IOW, RLIMIT_NPROC is ignored). The rest is fine. Thanks! --=20 Vasily Kulikov http://www.openwall.com - bringing security into open computing environ= ments -- To unsubscribe from this list: send the line "unsubscribe linux-man" in the body of a message to majordomo-u79uwXL29TY76Z2rM5mHXA@public.gmane.org More majordomo info at http://vger.kernel.org/majordomo-info.html