[AArch64][compiler-rt] Add Pointer Authentication support for VFORK.

[danielkiss]

Summary

The LR is stored to off-stack spill area where it is vulnerable.
"paciasp" add an auth code to the LR while the "autiasp" verifies that so
LR can't be modiifed on the spill area.

Test: build with -DCMAKE_C_FLAGS="-mbranch-protection=standard",
run on Armv8.3 capable hardware with PAuth.

[eugenis]

LGTM

[ostannard]

This patch was causing failures on the AArch64 buildbots, because the compilers they use (clang 10.0 and gcc 9.3.0) don't accept these instructions when targeting armv8.0-a. I've committed https://reviews.llvm.org/rG0070c9ee2d2ce8 to use the hint mnemonic instead for these, which is supported by all aarch64 assemblers.

compiler-rt/lib/sanitizer_common/sanitizer_common_interceptors_vfork_aarch64.inc.S

 #if defined(__aarch64__) && defined(__linux__)
 
 #include "sanitizer_common/sanitizer_asm.h"
+#include "builtins/assembly.h"
 
 ASM_HIDDEN(COMMON_INTERCEPTOR_SPILL_AREA)
 
 .comm _ZN14__interception10real_vforkE,8,8
 .globl ASM_WRAPPER_NAME(vfork)
 ASM_TYPE_FUNCTION(ASM_WRAPPER_NAME(vfork))
 ASM_WRAPPER_NAME(vfork):
         // Save x30 in the off-stack spill area.
+        paciasp
         stp     xzr, x30, [sp, #-16]!
         bl      COMMON_INTERCEPTOR_SPILL_AREA
         ldp     xzr, x30, [sp], 16
         str     x30, [x0]
 
         // Call real vfork. This may return twice. User code that runs between the first and the second return
         // may clobber the stack frame of the interceptor; that's why it does not have a frame.
         adrp    x0, _ZN14__interception10real_vforkE
         ldr     x0, [x0, :lo12:_ZN14__interception10real_vforkE]
         blr     x0
 
         stp     x0, xzr, [sp, #-16]!
         cmp     x0, #0
         b.eq   .L_exit
 
         // x0 != 0 => parent process. Clear stack shadow.
         add    x0, sp, #16
         bl     COMMON_INTERCEPTOR_HANDLE_VFORK
 
 .L_exit:
         // Restore x30.
         bl     COMMON_INTERCEPTOR_SPILL_AREA
         ldr    x30, [x0]
         ldp    x0, xzr, [sp], 16
+        autiasp
 
         ret
 ASM_SIZE(vfork)
 
 .weak vfork
 .set vfork, ASM_WRAPPER_NAME(vfork)
 
+GNU_PROPERTY_BTI_PAC
+
 #endif