Re: [PATCH] x86/uaccess: Avoid barrier_nospec() in copy_from_user()

[Josh Poimboeuf <jpoimboe@kernel.org>]

On Sun, Oct 20, 2024 at 03:59:25PM -0700, Linus Torvalds wrote:
> On Sun, 20 Oct 2024 at 15:44, Josh Poimboeuf <jpoimboe@kernel.org> wrote:
> >
> > Anyway, I'd really like to make forward progress on getting rid of the
> > LFENCEs in copy_from_user() and __get_user(), so until if/when we hear
> > back from both vendors, how about we avoid noncanonical exceptions
> > altogether (along with the edge cases mentioned above) and do something
> > like the below?
> 
> That doesn't work for LAM at _all_.

Argh, ok.

> So at a minimum, you need to then say "for LAM enabled CPU's we do the
> 'shift sign bit' trick".

Something like below to wipe out the LAM bits beforehand?

I'm probably overlooking something else as there are a lot of annoying
details here...

> Hopefully any LAM-capable CPU doesn't have this issue?
> 
> And I still think that clac/stac has to serialize with surrounding
> memory operations, making this all moot.

Until it's s/think/know/ can we please put something in place?


#define FORCE_CANONICAL								\
	ALTERNATIVE_2								\
	"shl $(64 - 48), %rdx",							\
	"shl $(64 - 57), %rdx", X86_FEATURE_LA57,				\
	"", ALT_NOT(X86_FEATURE_LAM)

#ifdef CONFIG_X86_5LEVEL
#define LOAD_TASK_SIZE_MINUS_N(n) \
	ALTERNATIVE __stringify(mov $((1 << 47) - 4096 - (n)),%rdx), \
		    __stringify(mov $((1 << 56) - 4096 - (n)),%rdx), X86_FEATURE_LA57
#else
#define LOAD_TASK_SIZE_MINUS_N(n) \
	mov $(TASK_SIZE_MAX - (n)),%_ASM_DX
#endif

.macro check_range size
.if IS_ENABLED(CONFIG_X86_64)
	FORCE_CANONICAL
	/* If above TASK_SIZE_MAX, convert to all 1's */
	LOAD_TASK_SIZE_MINUS_N(size-1)
	cmp %rax, %rdx
	sbb %rdx, %rdx
	or  %rdx, %rax