linux-kernel - Re: [PATCH] x86: Allow user accesses to the base of the guard page

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Message-ID: <CAHk-=wgT4E+6YO0-SvdD=Gh6wSuUB3Bq_qOiErfZkobQ9gn6=Q@mail.gmail.com>
Date: Sat, 23 Nov 2024 11:02:41 -0800
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: David Laight <David.Laight@...lab.com>
Cc: Andrew Cooper <andrew.cooper3@...rix.com>, "bp@...en8.de" <bp@...en8.de>, 
	Josh Poimboeuf <jpoimboe@...nel.org>, "x86@...nel.org" <x86@...nel.org>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, Arnd Bergmann <arnd@...nel.org>, 
	Mikel Rychliski <mikel@...elr.com>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] x86: Allow user accesses to the base of the guard page

On Sat, 23 Nov 2024 at 10:48, David Laight <David.Laight@...lab.com> wrote:
>
> In that case access_ok(ptr, size) will check that 'ptr + size'
> is a valid user address -

The point of USER_PTR_MAX is that the size never matters and we never
check it. So the "-1" is basically just the minimal size.

And the code does actually depend on the fact that the access has to
start *before* the boundary to work.

Now, we do have that whole "at least PAGE_SIZE of guard page", and so
the 1-byte minimal size doesn't actually matter, but I don't see the
point of the change.

In particular, I don't see when it would matter to do access_ok(ptr,
0) in the first place. Who does that, and why would it make any sense?

               Linus