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

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Message-ID: <2195976.7Z3S40VBb9@basin>
Date: Sat, 23 Nov 2024 19:24:21 -0500
From: Mikel Rychliski <mikel@...elr.com>
To: David Laight <David.Laight@...lab.com>,
 Linus Torvalds <torvalds@...ux-foundation.org>
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>, 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 Saturday, November 23, 2024 6:44:34 P.M. EST you wrote:
> There's a difference between "valid" and "we care".
> 
> This is way past that case. The only possible reason for that
> zero-byte thing at the end of the address space is somebody actively
> looking for some edge case, not a real use.

access_ok() for x86_64 checks the validity of the byte one past the end of the 
requested buffer, even if that buffer is non-zero.

I ran into this in kernels that include 86e6b1547b3d0 with a BPF program that 
grabs the bottom of the user stack in PAGE_SIZE chunks. Reading the final page 
of user space returns -EFAULT now because the access_ok() check fails.

I've been working around with this:
https://lore.kernel.org/lkml/20241109210313.440495-1-mikel@mikelr.com/