Re: ia64 exceptions (Re: [RFC][CFT][PATCHSET v1] uaccess unification)

Al Viro
2017-04-05T08:09:05Z

On Wed, Apr 05, 2017 at 06:05:08AM +0100, Al Viro wrote:

> Speaking of ia64: copy_user.S contains the following oddity:
> 2:
> EX(.failure_in3,(p16) ld8 val1[0]=[src1],16)
> (p16) ld8 val2[0]=[src2],16
>
> src1 is 16-byte aligned, src2 is src1 + 8.
>
> What guarantees that we can't race with e.g. TLB shootdown from a thread on
> another CPU, ending up with the second insn taking a fault and oopsing?
>
> AFAICS, other places where we have such pairs of loads or stores (e.g.
> EX(.ex_handler, (p16) ld8 r34=[src0],16)
> EK(.ex_handler, (p16) ld8 r38=[src1],16)
> in the memcpy_mck.S counterpart of that code) both have exception table
> entries associated with them.
>
> Is that one intentional and correct for some subtle reason, or is it a very
> narrow race on the hardware nobody gives a damn anymore? It is pre-mckinley
> stuff, after all...

Actually, the piece immediately after that one is worse. By that point,
we have
* checked that len is large enough to be worth bothering with word
copies. Fine.
* checked that src and dst have the same remainder modulo 8.
* copied until src is a multiple of 16, incrementing src and dst
by the same amount.
* prepared for copying in multiples of 16 bytes
* set src2 and dst2 8 bytes past src1 and dst1 resp.
and now we have a pipelined loop with
EX(.failure_in3,(p16) ld8 val1[0]=[src1],16)
(p16) ld8 val2[0]=[src2],16

EX(.failure_out, (EPI) st8 [dst1]=val1[PIPE_DEPTH-1],16)
(EPI) st8 [dst2]=val2[PIPE_DEPTH-1],16
for body. Now, consider the following case:

* to is 8 bytes before the end of user page, next page is unmapped
* from is at the beginning of kernel page
* len is simply PAGE_SIZE

and we call copy_to_user(). All the preparation work won't read or write
anything - all alignments are fine. src1 and src2 are kernel page and
kernel page + 8 resp.; dst1 is 8 bytes before the end of user page, dst2
is at the beginning of unmapped user page. No loads are going to fail;
the first store into dst1 won't fail either. The *second* store - one to
dst2 will not just fail, it'll oops.

<goes to test>

... and sure enough, on generic kernel (CONFIG_ITANIUM) that yields a nice
shiny oops at precisely that insn.

We really need tests for uaccess primitives. That's not a recent regression,
BTW - it had been that way since 2.3.48-pre2, as far as I can see.