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x86: mm: remove 'sign' games from LAM untagged_addr*() macros

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The intent of the sign games was to not modify kernel addresses when
untagging them.  However, that had two issues:

 (a) it didn't actually work as intended, since the mask was calculated
     as 'addr >> 63' on an _unsigned_ address. So instead of getting a
     mask of all ones for kernel addresses, you just got '1'.

 (b) untagging a kernel address isn't actually a valid operation anyway.

Now, (a) had originally been true for both 'untagged_addr()' and the
remote version of it, but had accidentally been fixed for the regular
version of untagged_addr() by commit e0bddc19ba ("x86/mm: Reduce
untagged_addr() overhead for systems without LAM").  That one rewrote
the shift to be part of the alternative asm code, and in the process
changed the unsigned shift into a signed 'sar' instruction.

And while it is true that we don't want to turn what looks like a kernel
address into a user address by masking off the high bit, that doesn't
need these sign masking games - all it needs is that the mm context
'untag_mask' value has the high bit set.

Which it always does.

So simplify the code by just removing the superfluous (and in the case
of untagged_addr_remote(), still buggy) sign bit games in the address
masking.

Acked-by: Dave Hansen <dave.hansen@intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Linus Torvalds 2023-05-03 09:38:58 -07:00
parent b9bd9f605c
commit 1dbc0a9515
1 changed files with 3 additions and 15 deletions
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18
arch/x86/include/asm/uaccess_64.h
View File

@ -15,25 +15,17 @@
#ifdef CONFIG_ADDRESS_MASKING
/*
* Mask out tag bits from the address.
*
* Magic with the 'sign' allows to untag userspace pointer without any branches
* while leaving kernel addresses intact.
*/
static inline unsigned long __untagged_addr(unsigned long addr)
{
long sign;
/*
* Refer tlbstate_untag_mask directly to avoid RIP-relative relocation
* in alternative instructions. The relocation gets wrong when gets
* copied to the target place.
*/
asm (ALTERNATIVE("",
"sar $63, %[sign]\n\t" /* user_ptr ? 0 : -1UL */
"or %%gs:tlbstate_untag_mask, %[sign]\n\t"
"and %[sign], %[addr]\n\t", X86_FEATURE_LAM)
: [addr] "+r" (addr), [sign] "=r" (sign)
: "m" (tlbstate_untag_mask), "[sign]" (addr));
"and %%gs:tlbstate_untag_mask, %[addr]\n\t", X86_FEATURE_LAM)
: [addr] "+r" (addr) : "m" (tlbstate_untag_mask));
return addr;
}
@ -46,12 +38,8 @@ static inline unsigned long __untagged_addr(unsigned long addr)
static inline unsigned long __untagged_addr_remote(struct mm_struct *mm,
unsigned long addr)
{
long sign = addr >> 63;
mmap_assert_locked(mm);
addr &= (mm)->context.untag_mask | sign;
return addr;
return addr & (mm)->context.untag_mask;
}
#define untagged_addr_remote(mm, addr) ({ \