Hello Vasiliy, On 05/26/2014 08:11 PM, Vasiliy Kulikov wrote: > Hi Michael, > > On Wed, May 21, 2014 at 20:12 +0200, Michael Kerrisk (man-pages) wrote: >> Vasily (and Motohiro), >> >> Sometime ago, Motohiro raised a documentation bug >> ( https://bugzilla.kernel.org/show_bug.cgi?id=42704 ) which >> relates to your commit 72fa59970f8698023045ab0713d66f3f4f96945c >> ("move RLIMIT_NPROC check from set_user() to do_execve_common()") >> >> 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. >> >> Thank you, >> >> Michael >> >> >> ERRORS >> EAGAIN (since Linux 3.1) >> Having changed its real UID using one of the set*uid() >> calls, the caller was—and is now still—above its >> RLIMIT_NPROC resource limit (see setrlimit(2)). For a >> more detailed explanation of this error, see NOTES. >> >> 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. >> >> 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. >> >> Since Linux 3.1, the scenario just described no longer causes >> the set*uid() call to fail, because it too often led to secu‐ >> rity holes because buggy applications didn't check the return >> status and assumed that—if the caller had root privileges—the >> 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—namely, fork(2)+ set*uid()+ >> execve(2). >> >> 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‐ >> ceeds. > > Probably explicitly state that NPROC check on execve() is processed only > 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). Yes, good idea. I'll add some words to make that clearer. > The rest is fine. Thanks for checking it! Cheers, Michael -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/ -- To unsubscribe from this list: send the line "unsubscribe linux-man" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
