Re: [PATCH v2 1/6] fs/readdir: Fix filldir() and filldir64() use of user_access_begin()

[Michael Ellerman]

Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> writes:
> On Thu, Jan 23, 2020 at 4:00 AM Michael Ellerman <mpe@xxxxxxxxxxxxxx> wrote:
>>
>> So I guess I'll wait and see what happens with patch 1.
>
> I've committed my fixes to filldir[64]() directly - they really were
> fixing me being lazy about the range, and the name length checking
> really is a theoretical "access wrong user space pointer" issue with
> corrupted filesystems regardless (even though I suspect it's entirely
> theoretical - even a corrupt filesystem hopefully won't be passing in
> negative directory entry lengths or something like that).

Great, thanks.

> The "pass in read/write" part I'm not entirely convinced about.
> Honestly, if this is just for ppc32 and nobody else really needs it,
> make the ppc32s thing always just enable both user space reads and
> writes. That's the semantics for x86 and arm as is, I'm not convinced
> that we should complicate this for a legacy platform.

We can use the read/write info on Power9 too. That's a niche platform
but hopefully not legacy status yet :P

But it's entirely optional, as you say we can just enable read/write if
we aren't passed the read/write info from the upper-level API.

I think our priority should be getting objtool going on powerpc to check
our user access regions are well contained. Once we have that working
maybe then we can look at plumbing the direction through
user_access_begin() etc.

cheers