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2016-01-20ptrace: use fsuid, fsgid, effective creds for fs access checksJann Horn1-2/+2
By checking the effective credentials instead of the real UID / permitted capabilities, ensure that the calling process actually intended to use its credentials. To ensure that all ptrace checks use the correct caller credentials (e.g. in case out-of-tree code or newly added code omits the PTRACE_MODE_*CREDS flag), use two new flags and require one of them to be set. The problem was that when a privileged task had temporarily dropped its privileges, e.g. by calling setreuid(0, user_uid), with the intent to perform following syscalls with the credentials of a user, it still passed ptrace access checks that the user would not be able to pass. While an attacker should not be able to convince the privileged task to perform a ptrace() syscall, this is a problem because the ptrace access check is reused for things in procfs. In particular, the following somewhat interesting procfs entries only rely on ptrace access checks: /proc/$pid/stat - uses the check for determining whether pointers should be visible, useful for bypassing ASLR /proc/$pid/maps - also useful for bypassing ASLR /proc/$pid/cwd - useful for gaining access to restricted directories that contain files with lax permissions, e.g. in this scenario: lrwxrwxrwx root root /proc/13020/cwd -> /root/foobar drwx------ root root /root drwxr-xr-x root root /root/foobar -rw-r--r-- root root /root/foobar/secret Therefore, on a system where a root-owned mode 6755 binary changes its effective credentials as described and then dumps a user-specified file, this could be used by an attacker to reveal the memory layout of root's processes or reveal the contents of files he is not allowed to access (through /proc/$pid/cwd). [akpm@linux-foundation.org: fix warning] Signed-off-by: Jann Horn <jann@thejh.net> Acked-by: Kees Cook <keescook@chromium.org> Cc: Casey Schaufler <casey@schaufler-ca.com> Cc: Oleg Nesterov <oleg@redhat.com> Cc: Ingo Molnar <mingo@redhat.com> Cc: James Morris <james.l.morris@oracle.com> Cc: "Serge E. Hallyn" <serge.hallyn@ubuntu.com> Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com> Cc: Andy Lutomirski <luto@kernel.org> Cc: Al Viro <viro@zeniv.linux.org.uk> Cc: "Eric W. Biederman" <ebiederm@xmission.com> Cc: Willy Tarreau <w@1wt.eu> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2014-09-10kcmp: fix standard comparison bugRasmus Villemoes1-3/+4
The C operator <= defines a perfectly fine total ordering on the set of values representable in a long. However, unlike its namesake in the integers, it is not translation invariant, meaning that we do not have "b <= c" iff "a+b <= a+c" for all a,b,c. This means that it is always wrong to try to boil down the relationship between two longs to a question about the sign of their difference, because the resulting relation [a LEQ b iff a-b <= 0] is neither anti-symmetric or transitive. The former is due to -LONG_MIN==LONG_MIN (take any two a,b with a-b = LONG_MIN; then a LEQ b and b LEQ a, but a != b). The latter can either be seen observing that x LEQ x+1 for all x, implying x LEQ x+1 LEQ x+2 ... LEQ x-1 LEQ x; or more directly with the simple example a=LONG_MIN, b=0, c=1, for which a-b < 0, b-c < 0, but a-c > 0. Note that it makes absolutely no difference that a transmogrying bijection has been applied before the comparison is done. In fact, had the obfuscation not been done, one could probably not observe the bug (assuming all values being compared always lie in one half of the address space, the mathematical value of a-b is always representable in a long). As it stands, one can easily obtain three file descriptors exhibiting the non-transitivity of kcmp(). Side note 1: I can't see that ensuring the MSB of the multiplier is set serves any purpose other than obfuscating the obfuscating code. Side note 2: #include <stdio.h> #include <stdlib.h> #include <string.h> #include <fcntl.h> #include <unistd.h> #include <assert.h> #include <sys/syscall.h> enum kcmp_type { KCMP_FILE, KCMP_VM, KCMP_FILES, KCMP_FS, KCMP_SIGHAND, KCMP_IO, KCMP_SYSVSEM, KCMP_TYPES, }; pid_t pid; int kcmp(pid_t pid1, pid_t pid2, int type, unsigned long idx1, unsigned long idx2) { return syscall(SYS_kcmp, pid1, pid2, type, idx1, idx2); } int cmp_fd(int fd1, int fd2) { int c = kcmp(pid, pid, KCMP_FILE, fd1, fd2); if (c < 0) { perror("kcmp"); exit(1); } assert(0 <= c && c < 3); return c; } int cmp_fdp(const void *a, const void *b) { static const int normalize[] = {0, -1, 1}; return normalize[cmp_fd(*(int*)a, *(int*)b)]; } #define MAX 100 /* This is plenty; I've seen it trigger for MAX==3 */ int main(int argc, char *argv[]) { int r, s, count = 0; int REL[3] = {0,0,0}; int fd[MAX]; pid = getpid(); while (count < MAX) { r = open("/dev/null", O_RDONLY); if (r < 0) break; fd[count++] = r; } printf("opened %d file descriptors\n", count); for (r = 0; r < count; ++r) { for (s = r+1; s < count; ++s) { REL[cmp_fd(fd[r], fd[s])]++; } } printf("== %d\t< %d\t> %d\n", REL[0], REL[1], REL[2]); qsort(fd, count, sizeof(fd[0]), cmp_fdp); memset(REL, 0, sizeof(REL)); for (r = 0; r < count; ++r) { for (s = r+1; s < count; ++s) { REL[cmp_fd(fd[r], fd[s])]++; } } printf("== %d\t< %d\t> %d\n", REL[0], REL[1], REL[2]); return (REL[0] + REL[2] != 0); } Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk> Reviewed-by: Cyrill Gorcunov <gorcunov@openvz.org> "Eric W. Biederman" <ebiederm@xmission.com> Cc: <stable@vger.kernel.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2012-12-20kcmp: include linux/ptrace.hCyrill Gorcunov1-0/+1
This makes it compile on s390. After all the ptrace_may_access (which we use this file) is declared exactly in linux/ptrace.h. This is preparatory work to wire this syscall up on all archs. Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org> Signed-off-by: Alexander Kartashov <alekskartashov@parallels.com> Cc: Heiko Carstens <heiko.carstens@de.ibm.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2012-05-31syscalls, x86: add __NR_kcmp syscallCyrill Gorcunov1-0/+196
While doing the checkpoint-restore in the user space one need to determine whether various kernel objects (like mm_struct-s of file_struct-s) are shared between tasks and restore this state. The 2nd step can be solved by using appropriate CLONE_ flags and the unshare syscall, while there's currently no ways for solving the 1st one. One of the ways for checking whether two tasks share e.g. mm_struct is to provide some mm_struct ID of a task to its proc file, but showing such info considered to be not that good for security reasons. Thus after some debates we end up in conclusion that using that named 'comparison' syscall might be the best candidate. So here is it -- __NR_kcmp. It takes up to 5 arguments - the pids of the two tasks (which characteristics should be compared), the comparison type and (in case of comparison of files) two file descriptors. Lookups for pids are done in the caller's PID namespace only. At moment only x86 is supported and tested. [akpm@linux-foundation.org: fix up selftests, warnings] [akpm@linux-foundation.org: include errno.h] [akpm@linux-foundation.org: tweak comment text] Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org> Acked-by: "Eric W. Biederman" <ebiederm@xmission.com> Cc: Pavel Emelyanov <xemul@parallels.com> Cc: Andrey Vagin <avagin@openvz.org> Cc: KOSAKI Motohiro <kosaki.motohiro@gmail.com> Cc: Ingo Molnar <mingo@elte.hu> Cc: H. Peter Anvin <hpa@zytor.com> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Glauber Costa <glommer@parallels.com> Cc: Andi Kleen <andi@firstfloor.org> Cc: Tejun Heo <tj@kernel.org> Cc: Matt Helsley <matthltc@us.ibm.com> Cc: Pekka Enberg <penberg@kernel.org> Cc: Eric Dumazet <eric.dumazet@gmail.com> Cc: Vasiliy Kulikov <segoon@openwall.com> Cc: Alexey Dobriyan <adobriyan@gmail.com> Cc: Valdis.Kletnieks@vt.edu Cc: Michal Marek <mmarek@suse.cz> Cc: Frederic Weisbecker <fweisbec@gmail.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>