linux-kernel - Re: [patch 3/4] x86/futex: Use user_*_masked

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Message-ID: <288844da-400f-46f8-aa12-e0309424fb14@csgroup.eu>
Date: Tue, 26 Aug 2025 09:09:08 +0200
From: Christophe Leroy <christophe.leroy@...roup.eu>
To: Thomas Gleixner <tglx@...utronix.de>, LKML <linux-kernel@...r.kernel.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
 Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, x86@...nel.org,
 Peter Zijlstra <peterz@...radead.org>, Darren Hart <dvhart@...radead.org>,
 Davidlohr Bueso <dave@...olabs.net>, André Almeida
 <andrealmeid@...lia.com>, Alexander Viro <viro@...iv.linux.org.uk>,
 Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
 linux-fsdevel@...r.kernel.org
Subject: Re: [patch 3/4] x86/futex: Use user_*_masked_begin()



Le 13/08/2025 à 17:57, Thomas Gleixner a écrit :
> Replace the can_do_masked_user_access() conditional with the generic macro.
> 
> No functional change.
> 
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> Cc: x86@...nel.org
> ---
>   arch/x86/include/asm/futex.h |   12 ++++--------
>   1 file changed, 4 insertions(+), 8 deletions(-)
> 
> --- a/arch/x86/include/asm/futex.h
> +++ b/arch/x86/include/asm/futex.h
> @@ -48,9 +48,7 @@ do {								\
>   static __always_inline int arch_futex_atomic_op_inuser(int op, int oparg, int *oval,
>   		u32 __user *uaddr)
>   {
> -	if (can_do_masked_user_access())
> -		uaddr = masked_user_access_begin(uaddr);
> -	else if (!user_access_begin(uaddr, sizeof(u32)))
> +	if (!user_write_masked_begin(uaddr))

You are replacing a user_access_begin() by a macro that calls 
user_write_access_begin(). I means that reads will not be allowed, 
allthough arch_futex_atomic_op_inuser() performs read-then-write, so it 
requires a full read-write user access.

>   		return -EFAULT;
>   
>   	switch (op) {
> @@ -74,7 +72,7 @@ static __always_inline int arch_futex_at
>   		user_access_end();
>   		return -ENOSYS;
>   	}
> -	user_access_end();
> +	user_write_access_end();

Same, can't be changed to write-only, read permission is required as well.

>   	return 0;
>   Efault:
>   	user_access_end();
> @@ -86,9 +84,7 @@ static inline int futex_atomic_cmpxchg_i
>   {
>   	int ret = 0;
>   
> -	if (can_do_masked_user_access())
> -		uaddr = masked_user_access_begin(uaddr);
> -	else if (!user_access_begin(uaddr, sizeof(u32)))
> +	if (!user_write_masked_begin(uaddr))

Same, read access is also needed.

>   		return -EFAULT;
>   	asm volatile("\n"
>   		"1:\t" LOCK_PREFIX "cmpxchgl %3, %2\n"
> @@ -98,7 +94,7 @@ static inline int futex_atomic_cmpxchg_i
>   		: "r" (newval), "1" (oldval)
>   		: "memory"
>   	);
> -	user_access_end();
> +	user_write_access_end();

Same, read access is also needed.

>   	*uval = oldval;
>   	return ret;
>   }
> 
>