Hi Maciej,
On Sun, Jan 31, 2016 at 11:33:30PM +0000, Maciej W. Rozycki wrote:
> On Mon, 25 Jan 2016, James Hogan wrote:
>
> > Since commit 4c21b8fd8f14 ("MIPS: seccomp: Handle indirect system calls
> > (o32)"), syscall_get_arguments() attempts to handle o32 indirect syscall
> > arguments by incrementing both the start argument number and the number
> > of arguments to fetch. However only the start argument number needs to
> > be incremented. The number of arguments does not change, they're just
> > shifted up by one, and in fact the output array is provided by the
> > caller and is likely only n entries long, so reading more arguments
> > overflows the output buffer.
> [...]
> > diff --git a/arch/mips/include/asm/syscall.h b/arch/mips/include/asm/syscall.h
> > index 6499d93ae68d..47bc45a67e9b 100644
> > --- a/arch/mips/include/asm/syscall.h
> > +++ b/arch/mips/include/asm/syscall.h
> > @@ -101,10 +101,8 @@ static inline void syscall_get_arguments(struct task_struct *task,
> > /* O32 ABI syscall() - Either 64-bit with O32 or 32-bit */
> > if ((config_enabled(CONFIG_32BIT) ||
> > test_tsk_thread_flag(task, TIF_32BIT_REGS)) &&
> > - (regs->regs[2] == __NR_syscall)) {
> > + (regs->regs[2] == __NR_syscall))
> > i++;
> > - n++;
> > - }
> >
> > while (n--)
> > ret |= mips_get_syscall_arg(args++, task, regs, i++);
>
> What I think it really needs to do is to *decrease* the number of
> arguments, as we're throwing the syscall number away as not an argument to
> itself. So this looks like a typo to me, the expression was meant to be
> `n--' rather than `n++'. With the number of arguments unchanged, as in
> your proposed change, we're still reaching one word too far.
No, the caller asked for n args, thats what it should get. It doesn't
care whether the syscall was indirected or not.
The syscall doesn't have 1 less arg as a result of indirection. E.g.
What about system calls with 6 arguments, and hence 7 arguments
including syscall number argument when redirected? We'd get an
uninitialised 6th arg back when passing n=6.
Note scall32-o32.S sys_syscall shifts 7 arguments starting at a1 (I've
reordered code slightly):
move a0, a1 # shift argument registers
move a1, a2
move a2, a3
lw a3, 16(sp)
lw t4, 20(sp)
lw t5, 24(sp)
lw t6, 28(sp)
sw a0, PT_R4(sp) # .. and push back a0 - a3, some
sw a1, PT_R5(sp) # syscalls expect them there
sw a2, PT_R6(sp)
sw a3, PT_R7(sp)
sw t4, 16(sp)
sw t5, 20(sp)
sw t6, 24(sp)
So it takes args 0..2 from a1..a3, and 3..6 from stack.
I presume the handling of 7th arg after syscall number argument is for
fadvise64_64 and sync_file_range, which had badly designed arguments
(odd 64bit args requiring a word of padding).
Cheers
James
>
> Maciej
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