[PATCH] arm64: Apply dynamic shadow call stack patching in two passes

[Ard Biesheuvel]

Code patching for the dynamically enabled shadow call stack comes down
to finding PACIASP and AUTIASP instructions -which behave as NOPs on
cores that do not implement pointer authentication- and converting them
into shadow call stack pushes and pops, respectively.

Due to past bad experiences with the highly complex and overengineered
DWARF standard that describes the unwind metadata that we are using to
locate these instructions, let's make this patching logic a little bit
more robust so that any issues with the unwind metadata detected at boot
time can de dealt with gracefully.

The DWARF annotations that are used for this are emitted at function
granularity, and due to the fact that the instructions we are patching
will simply behave as NOPs if left unpatched, we can abort on errors as
long as we don't leave any functions in a half-patched state.

So do a dry run of each FDE frame (covering a single function) before
performing the actual patching, and give up if the DWARF metadata cannot
be understood.

Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
---
 arch/arm64/kernel/patch-scs.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/kernel/patch-scs.c b/arch/arm64/kernel/patch-scs.c
index 1b3da02d5b741bc3..a1fe4b4ff5917670 100644
--- a/arch/arm64/kernel/patch-scs.c
+++ b/arch/arm64/kernel/patch-scs.c
@@ -130,7 +130,8 @@ struct eh_frame {
 
 static int noinstr scs_handle_fde_frame(const struct eh_frame *frame,
 					bool fde_has_augmentation_data,
-					int code_alignment_factor)
+					int code_alignment_factor,
+					bool dry_run)
 {
 	int size = frame->size - offsetof(struct eh_frame, opcodes) + 4;
 	u64 loc = (u64)offset_to_ptr(&frame->initial_loc);
@@ -184,7 +185,8 @@ static int noinstr scs_handle_fde_frame(const struct eh_frame *frame,
 			break;
 
 		case DW_CFA_negate_ra_state:
-			scs_patch_loc(loc - 4);
+			if (!dry_run)
+				scs_patch_loc(loc - 4);
 			break;
 
 		case 0x40 ... 0x7f:
@@ -235,9 +237,12 @@ int noinstr scs_patch(const u8 eh_frame[], int size)
 		} else {
 			ret = scs_handle_fde_frame(frame,
 						   fde_has_augmentation_data,
-						   code_alignment_factor);
+						   code_alignment_factor,
+						   true);
 			if (ret)
 				return ret;
+			scs_handle_fde_frame(frame, fde_has_augmentation_data,
+					     code_alignment_factor, false);
 		}
 
 		p += sizeof(frame->size) + frame->size;
-- 
2.35.1