Re: [RFC][PATCH v2 4/5] x86/uaccess: Implement unsafe_try_cmpxchg_user()

Sean Christopherson
2022-01-27T02:17:26Z

On Thu, Jan 20, 2022, Peter Zijlstra wrote:
> Do try_cmpxchg() loops on userspace addresses.
>
> Cc: Sean Christopherson <seanjc@google.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
> arch/x86/include/asm/uaccess.h | 67 +++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 67 insertions(+)
>
> --- a/arch/x86/include/asm/uaccess.h
> +++ b/arch/x86/include/asm/uaccess.h
> @@ -342,6 +342,24 @@ do { \
> : [umem] "m" (__m(addr)) \
> : : label)
>
> +#define __try_cmpxchg_user_asm(itype, ltype, _ptr, _pold, _new, label) ({ \
> + bool success; \
> + __typeof__(_ptr) _old = (__typeof__(_ptr))(_pold); \
> + __typeof__(*(_ptr)) __old = *_old; \
> + __typeof__(*(_ptr)) __new = (_new); \
> + asm_volatile_goto("\n" \
> + "1: " LOCK_PREFIX "cmpxchg"itype" %[new], %[ptr]\n"\
> + _ASM_EXTABLE_UA(1b, %l[label]) \
> + : CC_OUT(z) (success), \
> + [ptr] "+m" (*_ptr), \
> + [old] "+a" (__old) \
> + : [new] ltype (__new) \
> + : "memory", "cc" \

IIUC, the "cc" clobber is unnecessary as CONFIG_CC_HAS_ASM_GOTO_OUTPUT=y implies
__GCC_ASM_FLAG_OUTPUTS__=y, i.e. CC_OUT() will resolve to "=@cc".

> + : label); \
> + if (unlikely(!success)) \
> + *_old = __old; \
> + likely(success); })
> +
> #else // !CONFIG_CC_HAS_ASM_GOTO_OUTPUT

...

> +extern void __try_cmpxchg_user_wrong_size(void);
> +
> +#define unsafe_try_cmpxchg_user(_ptr, _oldp, _nval, _label) ({ \
> + __typeof__(*(_ptr)) __ret; \

This should probably be a bool, the return from the lower level helpers is a bool
that's true if the exchange succeed. Declaring the type of the target implies
that they return the raw result, which is confusing.

> + switch (sizeof(__ret)) { \
> + case 1: __ret = __try_cmpxchg_user_asm("b", "q", \
> + (_ptr), (_oldp), \
> + (_nval), _label); \
> + break; \
> + case 2: __ret = __try_cmpxchg_user_asm("w", "r", \
> + (_ptr), (_oldp), \
> + (_nval), _label); \
> + break; \
> + case 4: __ret = __try_cmpxchg_user_asm("l", "r", \
> + (_ptr), (_oldp), \
> + (_nval), _label); \
> + break; \
> + case 8: __ret = __try_cmpxchg_user_asm("q", "r", \
> + (_ptr), (_oldp), \
> + (_nval), _label); \

Doh, I should have specified that KVM needs 8-byte CMPXCHG on 32-bit kernels due
to using it to atomically update guest PAE PTEs and LTR descriptors (yay).

Also, KVM's use case isn't a tight loop, how gross would it be to add a slightly
less unsafe version that does __uaccess_begin_nospec()? KVM pre-checks the address
way ahead of time, so the access_ok() check can be omitted. Alternatively, KVM
could add its own macro, but that seems a little silly. E.g. somethign like this,
though I don't think this is correct (something is getting inverted somewhere and
the assembly output is a nightmare):

/* "Returns" 0 on success, 1 on failure, -EFAULT if the access faults. */
#define ___try_cmpxchg_user(_ptr, _oldp, _nval, _label) ({ \
int ____ret = -EFAULT; \
__uaccess_begin_nospec(); \
____ret = !unsafe_try_cmpxchg_user(_ptr, _oldp, _nval, _label); \
_label: \
__uaccess_end(); \
____ret; \
})

Lastly, assuming I get my crap working, mind if I post a variant (Cc'd to stable@) in
the context of KVM series? Turns out KVM has an ugly bug where it completely
botches the pfn calculation of memory it remaps and accesses[*], the easiest fix
is to switch to __try_cmpxchg_user() and purge the nastiness.

[*] https://lore.kernel.org/all/20220124172633.103323-1-tadeusz.struk@linaro.org