• Sergei Trofimovich's avatar
    alpha: fix page fault handling for r16-r18 targets · 1d8b2030
    Sergei Trofimovich authored
    commit 491af60ffb848b59e82f7c9145833222e0bf27a5 upstream.
    
    Fix page fault handling code to fixup r16-r18 registers.
    Before the patch code had off-by-two registers bug.
    This bug caused overwriting of ps,pc,gp registers instead
    of fixing intended r16,r17,r18 (see `struct pt_regs`).
    
    More details:
    
    Initially Dmitry noticed a kernel bug as a failure
    on strace test suite. Test passes unmapped userspace
    pointer to io_submit:
    
    ```c
        #include <err.h>
        #include <unistd.h>
        #include <sys/mman.h>
        #include <asm/unistd.h>
        int main(void)
        {
            unsigned long ctx = 0;
            if (syscall(__NR_io_setup, 1, &ctx))
                err(1, "io_setup");
            const size_t page_size = sysconf(_SC_PAGESIZE);
            const size_t size = page_size * 2;
            void *ptr = mmap(NULL, size, PROT_READ | PROT_WRITE,
                             MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
            if (MAP_FAILED == ptr)
                err(1, "mmap(%zu)", size);
            if (munmap(ptr, size))
                err(1, "munmap");
            syscall(__NR_io_submit, ctx, 1, ptr + page_size);
            syscall(__NR_io_destroy, ctx);
            return 0;
        }
    ```
    
    Running this test causes kernel to crash when handling page fault:
    
    ```
        Unable to handle kernel paging request at virtual address ffffffffffff9468
        CPU 3
        aio(26027): Oops 0
        pc = [<fffffc00004eddf8>]  ra = [<fffffc00004edd5c>]  ps = 0000    Not tainted
        pc is at sys_io_submit+0x108/0x200
        ra is at sys_io_submit+0x6c/0x200
        v0 = fffffc00c58e6300  t0 = fffffffffffffff2  t1 = 000002000025e000
        t2 = fffffc01f159fef8  t3 = fffffc0001009640  t4 = fffffc0000e0f6e0
        t5 = 0000020001002e9e  t6 = 4c41564e49452031  t7 = fffffc01f159c000
        s0 = 0000000000000002  s1 = 000002000025e000  s2 = 0000000000000000
        s3 = 0000000000000000  s4 = 0000000000000000  s5 = fffffffffffffff2
        s6 = fffffc00c58e6300
        a0 = fffffc00c58e6300  a1 = 0000000000000000  a2 = 000002000025e000
        a3 = 00000200001ac260  a4 = 00000200001ac1e8  a5 = 0000000000000001
        t8 = 0000000000000008  t9 = 000000011f8bce30  t10= 00000200001ac440
        t11= 0000000000000000  pv = fffffc00006fd320  at = 0000000000000000
        gp = 0000000000000000  sp = 00000000265fd174
        Disabling lock debugging due to kernel taint
        Trace:
        [<fffffc0000311404>] entSys+0xa4/0xc0
    ```
    
    Here `gp` has invalid value. `gp is s overwritten by a fixup for the
    following page fault handler in `io_submit` syscall handler:
    
    ```
        __se_sys_io_submit
        ...
            ldq     a1,0(t1)
            bne     t0,4280 <__se_sys_io_submit+0x180>
    ```
    
    After a page fault `t0` should contain -EFALUT and `a1` is 0.
    Instead `gp` was overwritten in place of `a1`.
    
    This happens due to a off-by-two bug in `dpf_reg()` for `r16-r18`
    (aka `a0-a2`).
    
    I think the bug went unnoticed for a long time as `gp` is one
    of scratch registers. Any kernel function call would re-calculate `gp`.
    
    Dmitry tracked down the bug origin back to 2.1.32 kernel version
    where trap_a{0,1,2} fields were inserted into struct pt_regs.
    And even before that `dpf_reg()` contained off-by-one error.
    
    Cc: Richard Henderson <rth@twiddle.net>
    Cc: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
    Cc: linux-alpha@vger.kernel.org
    Cc: linux-kernel@vger.kernel.org
    Reported-and-reviewed-by: default avatar"Dmitry V. Levin" <ldv@altlinux.org>
    Cc: stable@vger.kernel.org # v2.1.32+
    Bug: https://bugs.gentoo.org/672040Signed-off-by: default avatarSergei Trofimovich <slyfox@gentoo.org>
    Signed-off-by: default avatarMatt Turner <mattst88@gmail.com>
    Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
    1d8b2030
fault.c 6.25 KB