From: David Laight <David.Laight@ACULAB.COM>
To: 'Josh Poimboeuf' <jpoimboe@redhat.com>,
Al Viro <viro@zeniv.linux.org.uk>
Cc: "x86@kernel.org" <x86@kernel.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
Linus Torvalds <torvalds@linux-foundation.org>,
Will Deacon <will@kernel.org>,
Dan Williams <dan.j.williams@intel.com>,
Andrea Arcangeli <aarcange@redhat.com>,
"Waiman Long" <longman@redhat.com>,
Peter Zijlstra <peterz@infradead.org>,
"Thomas Gleixner" <tglx@linutronix.de>,
Andrew Cooper <andrew.cooper3@citrix.com>,
Andy Lutomirski <luto@kernel.org>, Christoph Hellwig <hch@lst.de>,
"Mark Rutland" <mark.rutland@arm.com>,
Borislav Petkov <bp@alien8.de>
Subject: RE: [PATCH v4 3/4] x86/uaccess: Use pointer masking to limit uaccess speculation
Date: Wed, 5 May 2021 08:48:48 +0000 [thread overview]
Message-ID: <2f75c496ac774444b75ff808854b8e5f@AcuMS.aculab.com> (raw)
In-Reply-To: <5ba93cdbf35ab40264a9265fc24575a9b2f813b3.1620186182.git.jpoimboe@redhat.com>
From: Josh Poimboeuf
> Sent: 05 May 2021 04:55
>
> The x86 uaccess code uses barrier_nospec() in various places to prevent
> speculative dereferencing of user-controlled pointers (which might be
> combined with further gadgets or CPU bugs to leak data).
...
> Remove existing barrier_nospec() usage, and instead do user pointer
> masking, throughout the x86 uaccess code. This is similar to what arm64
> is already doing with uaccess_mask_ptr().
...
> diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
> index fb75657b5e56..ebe9ab46b183 100644
> --- a/arch/x86/include/asm/uaccess.h
> +++ b/arch/x86/include/asm/uaccess.h
> @@ -66,12 +66,35 @@ static inline bool pagefault_disabled(void);
> * Return: true (nonzero) if the memory block may be valid, false (zero)
> * if it is definitely invalid.
> */
> -#define access_ok(addr, size) \
> +#define access_ok(addr, size) \
> ({ \
> WARN_ON_IN_IRQ(); \
> likely(!__range_not_ok(addr, size, TASK_SIZE_MAX)); \
> })
>
> +/*
> + * Sanitize a user pointer such that it becomes NULL if it's not a valid user
> + * pointer. This prevents speculatively dereferencing a user-controlled
> + * pointer to kernel space if access_ok() speculatively returns true. This
> + * should be done *after* access_ok(), to avoid affecting error handling
> + * behavior.
> + */
> +#define mask_user_ptr(ptr) \
> +({ \
> + unsigned long _ptr = (__force unsigned long)ptr; \
> + unsigned long mask; \
> + \
> + asm volatile("cmp %[max], %[_ptr]\n\t" \
> + "sbb %[mask], %[mask]\n\t" \
> + : [mask] "=r" (mask) \
> + : [_ptr] "r" (_ptr), \
> + [max] "r" (TASK_SIZE_MAX) \
> + : "cc"); \
> + \
> + mask &= _ptr; \
> + ((typeof(ptr)) mask); \
> +})
> +
access_ok() and mask_user_ptr() are doing much the same check.
Is there scope for making access_ok() return the masked pointer?
So the canonical calling code would be:
uptr = access_ok(uptr, size);
if (!uptr)
return -EFAULT;
This would error requests for address 0 earlier - but I don't
believe they are ever valid in Linux.
(Some historic x86 a.out formats did load to address 0.)
Clearly for a follow up patch.
David
-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)
next prev parent reply other threads:[~2021-05-05 8:48 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-05-05 3:54 [PATCH v4 0/4] x86/uaccess: Use pointer masking to limit uaccess speculation Josh Poimboeuf
2021-05-05 3:54 ` [PATCH v4 1/4] uaccess: Always inline strn*_user() helper functions Josh Poimboeuf
2021-05-05 3:54 ` [PATCH v4 2/4] uaccess: Fix __user annotations for copy_mc_to_user() Josh Poimboeuf
2021-05-05 3:54 ` [PATCH v4 3/4] x86/uaccess: Use pointer masking to limit uaccess speculation Josh Poimboeuf
2021-05-05 8:48 ` David Laight [this message]
2021-05-05 13:19 ` Josh Poimboeuf
2021-05-05 13:51 ` David Laight
2021-05-05 18:32 ` Linus Torvalds
2021-05-06 7:57 ` David Laight
2021-05-05 14:25 ` Mark Rutland
2021-05-05 14:48 ` Josh Poimboeuf
2021-05-05 14:49 ` David Laight
2021-05-05 15:45 ` Mark Rutland
2021-05-05 16:55 ` Andy Lutomirski
2021-05-06 8:36 ` David Laight
2021-05-06 12:05 ` Christoph Hellwig
2021-06-02 17:11 ` Sean Christopherson
2021-06-02 20:11 ` Josh Poimboeuf
2021-05-05 3:54 ` [PATCH v4 4/4] x86/nospec: Remove barrier_nospec() Josh Poimboeuf
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