Commit 1d8b2030 authored by Sergei Trofimovich's avatar Sergei Trofimovich Committed by Greg Kroah-Hartman
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alpha: fix page fault handling for r16-r18 targets

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:

    #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
    [<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:

        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 <>
Cc: Ivan Kokshaysky <>
Reported-and-reviewed-by: default avatar"Dmitry V. Levin" <>
Cc: # v2.1.32+

Signed-off-by: default avatarSergei Trofimovich <>
Signed-off-by: default avatarMatt Turner <>
Signed-off-by: default avatarGreg Kroah-Hartman <>
parent fd08513b
......@@ -77,7 +77,7 @@ __load_new_mm_context(struct mm_struct *next_mm)
/* Macro for exception fixup code to access integer registers. */
#define dpf_reg(r) \
(((unsigned long *)regs)[(r) <= 8 ? (r) : (r) <= 15 ? (r)-16 : \
(r) <= 18 ? (r)+8 : (r)-10])
(r) <= 18 ? (r)+10 : (r)-10])
asmlinkage void
do_page_fault(unsigned long address, unsigned long mmcsr,
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