RE: [PATCH v2] x86: Allow user accesses to the base of the guard page

[David Laight <David.Laight@ACULAB.COM>]

CC stable.

This needs picking up for 6.12

Head commit 573f45a9f9a47 applied by Linus with a modified commit message.

	David

> -----Original Message-----
> From: David Laight
> Sent: 24 November 2024 15:39
> To: 'Linus Torvalds' <torvalds@linux-foundation.org>; 'Andrew Cooper' <andrew.cooper3@citrix.com>;
> 'bp@alien8.de' <bp@alien8.de>; 'Josh Poimboeuf' <jpoimboe@kernel.org>
> Cc: 'x86@kernel.org' <x86@kernel.org>; 'linux-kernel@vger.kernel.org' <linux-kernel@vger.kernel.org>;
> 'Arnd Bergmann' <arnd@kernel.org>; 'Mikel Rychliski' <mikel@mikelr.com>; 'Thomas Gleixner'
> <tglx@linutronix.de>; 'Ingo Molnar' <mingo@redhat.com>; 'Borislav Petkov' <bp@alien8.de>; 'Dave
> Hansen' <dave.hansen@linux.intel.com>; 'H. Peter Anvin' <hpa@zytor.com>
> Subject: [PATCH v2] x86: Allow user accesses to the base of the guard page
> 
> __access_ok() calls valid_user_address() with the address after
> the last byte of the user buffer.
> It is valid for a buffer to end with the last valid user address
> so valid_user_address() must allow accesses to the base of the
> guard page.
> 
> Fixes: 86e6b1547b3d0 ("x86: fix user address masking non-canonical speculation issue")
> Signed-off-by: David Laight <david.laight@aculab.com>
> ---
> 
> v2: Rewritten commit message.
> 
>  arch/x86/kernel/cpu/common.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
> index 06a516f6795b..ca327cfa42ae 100644
> --- a/arch/x86/kernel/cpu/common.c
> +++ b/arch/x86/kernel/cpu/common.c
> @@ -2389,12 +2389,12 @@ void __init arch_cpu_finalize_init(void)
>  	alternative_instructions();
> 
>  	if (IS_ENABLED(CONFIG_X86_64)) {
> -		unsigned long USER_PTR_MAX = TASK_SIZE_MAX-1;
> +		unsigned long USER_PTR_MAX = TASK_SIZE_MAX;
> 
>  		/*
>  		 * Enable this when LAM is gated on LASS support
>  		if (cpu_feature_enabled(X86_FEATURE_LAM))
> -			USER_PTR_MAX = (1ul << 63) - PAGE_SIZE - 1;
> +			USER_PTR_MAX = (1ul << 63) - PAGE_SIZE;
>  		 */
>  		runtime_const_init(ptr, USER_PTR_MAX);
> 
> --
> 2.17.1

-
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