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: Matt Turner mattst88@gmail.com Cc: linux-alpha@vger.kernel.org Cc: linux-kernel@vger.kernel.org Reported-and-reviewed-by: "Dmitry V. Levin" ldv@altlinux.org Cc: stable@vger.kernel.org # v2.1.32+ Bug: https://bugs.gentoo.org/672040 Signed-off-by: Sergei Trofimovich slyfox@gentoo.org --- Changes since V1: - expanded bug origin tracked down by Dmitry - added proper Dmitry's email and reviwed by tags

arch/alpha/mm/fault.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/alpha/mm/fault.c b/arch/alpha/mm/fault.c index d73dc473fbb9..188fc9256baf 100644 --- a/arch/alpha/mm/fault.c +++ b/arch/alpha/mm/fault.c @@ -78,7 +78,7 @@ __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,