• Helge Deller's avatar
    parisc: Fix pagefault crash in unaligned __get_user() call · fcf55035
    Helge Deller authored
    commit 8b78f260 upstream.
    
    One of the debian buildd servers had this crash in the syslog without
    any other information:
    
     Unaligned handler failed, ret = -2
     clock_adjtime (pid 22578): Unaligned data reference (code 28)
     CPU: 1 PID: 22578 Comm: clock_adjtime Tainted: G  E  4.5.0-2-parisc64-smp #1 Debian 4.5.4-1
     task: 000000007d9960f8 ti: 00000001bde7c000 task.ti: 00000001bde7c000
    
          YZrvWESTHLNXBCVMcbcbcbcbOGFRQPDI
     PSW: 00001000000001001111100000001111 Tainted: G            E
     r00-03  000000ff0804f80f 00000001bde7c2b0 00000000402d2be8 00000001bde7c2b0
     r04-07  00000000409e1fd0 00000000fa6f7fff 00000001bde7c148 00000000fa6f7fff
     r08-11  0000000000000000 00000000ffffffff 00000000fac9bb7b 000000000002b4d4
     r12-15  000000000015241c 000000000015242c 000000000000002d 00000000fac9bb7b
     r16-19  0000000000028800 0000000000000001 0000000000000070 00000001bde7c218
     r20-23  0000000000000000 00000001bde7c210 0000000000000002 0000000000000000
     r24-27  0000000000000000 0000000000000000 00000001bde7c148 00000000409e1fd0
     r28-31  0000000000000001 00000001bde7c320 00000001bde7c350 00000001bde7c218
     sr00-03  0000000001200000 0000000001200000 0000000000000000 0000000001200000
     sr04-07  0000000000000000 0000000000000000 0000000000000000 0000000000000000
    
     IASQ: 0000000000000000 0000000000000000 IAOQ: 00000000402d2e84 00000000402d2e88
      IIR: 0ca0d089    ISR: 0000000001200000  IOR: 00000000fa6f7fff
      CPU:        1   CR30: 00000001bde7c000 CR31: ffffffffffffffff
      ORIG_R28: 00000002369fe628
      IAOQ[0]: compat_get_timex+0x2dc/0x3c0
      IAOQ[1]: compat_get_timex+0x2e0/0x3c0
      RP(r2): compat_get_timex+0x40/0x3c0
     Backtrace:
      [<00000000402d4608>] compat_SyS_clock_adjtime+0x40/0xc0
      [<0000000040205024>] syscall_exit+0x0/0x14
    
    This means the userspace program clock_adjtime called the clock_adjtime()
    syscall and then crashed inside the compat_get_timex() function.
    Syscalls should never crash programs, but instead return EFAULT.
    
    The IIR register contains the executed instruction, which disassebles
    into "ldw 0(sr3,r5),r9".
    This load-word instruction is part of __get_user() which tried to read the word
    at %r5/IOR (0xfa6f7fff). This means the unaligned handler jumped in.  The
    unaligned handler is able to emulate all ldw instructions, but it fails if it
    fails to read the source e.g. because of page fault.
    
    The following program reproduces the problem:
    
    #define _GNU_SOURCE
    #include <unistd.h>
    #include <sys/syscall.h>
    #include <sys/mman.h>
    
    int main(void) {
            /* allocate 8k */
            char *ptr = mmap(NULL, 2*4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);
            /* free second half (upper 4k) and make it invalid. */
            munmap(ptr+4096, 4096);
            /* syscall where first int is unaligned and clobbers into invalid memory region */
            /* syscall should return EFAULT */
            return syscall(__NR_clock_adjtime, 0, ptr+4095);
    }
    
    To fix this issue we simply need to check if the faulting instruction address
    is in the exception fixup table when the unaligned handler failed. If it
    is, call the fixup routine instead of crashing.
    
    While looking at the unaligned handler I found another issue as well: The
    target register should not be modified if the handler was unsuccessful.
    Signed-off-by: default avatarHelge Deller <deller@gmx.de>
    Cc: stable@vger.kernel.org
    Signed-off-by: default avatarWilly Tarreau <w@1wt.eu>
    fcf55035
unaligned.c 17.5 KB