arm64: expose user PAC bit positions via ptrace

When pointer authentication is in use, data/instruction pointers have a
number of PAC bits inserted into them. The number and position of these
bits depends on the configured TCR_ELx.TxSZ and whether tagging is
enabled. ARMv8.3 allows tagging to differ for instruction and data
pointers.

For userspace debuggers to unwind the stack and/or to follow pointer
chains, they need to be able to remove the PAC bits before attempting to
use a pointer.

This patch adds a new structure with masks describing the location of
the PAC bits in userspace instruction and data pointers (i.e. those
addressable via TTBR0), which userspace can query via PTRACE_GETREGSET.
By clearing these bits from pointers (and replacing them with the value
of bit 55), userspace can acquire the PAC-less versions.

This new regset is exposed when the kernel is built with (user) pointer
authentication support, and the address authentication feature is
enabled. Otherwise, the regset is hidden.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Kristina Martsenko <kristina.martsenko@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Ramana Radhakrishnan <ramana.radhakrishnan@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
[will: Fix to use vabits_user instead of VA_BITS and rename macro]
Signed-off-by: Will Deacon <will.deacon@arm.com>

1 files changed, 3 insertions, 0 deletions

diff --git a/arch/arm64/include/asm/memory.h b/arch/arm64/include/asm/memory.h
index 6747a3eddeb1..bd749033bc88 100644
--- a/arch/arm64/include/asm/memory.h
+++ b/arch/arm64/include/asm/memory.h
@@ -207,6 +207,9 @@ static inline unsigned long kaslr_offset(void)
 	return kimage_vaddr - KIMAGE_VADDR;
 }
 
+/* the actual size of a user virtual address */
+extern u64			vabits_user;
+
 /*
  * Allow all memory at the discovery stage. We will clip it later.
  */