[PATCH v1 2/2] arm64: Enable BTI for main executable as well as the interpreter

[Mark Brown]

Currently for dynamically linked ELF executables we only enable BTI for
the interpreter, expecting the interpreter to do this for the main
executable. This is a bit inconsistent since we do map main executable and
is causing issues with systemd's MemoryDenyWriteExecute feature which is
implemented using a seccomp filter which prevents setting PROT_EXEC on
already mapped memory and lacks the context to be able to detect that
memory is already mapped with PROT_EXEC.

Resolve this by checking the BTI property for the main executable and
enabling BTI if it is present when doing the initial mapping. This does
mean that we may get more code with BTI enabled if running on a system
without BTI support in the dynamic linker, this is expected to be a safe
configuration and testing seems to confirm that. It also reduces the
flexibility userspace has to disable BTI but it is expected that for cases
where there are problems which require BTI to be disabled it is more likely
that it will need to be disabled on a system level.

Signed-off-by: Mark Brown <broonie at kernel.org>
---
 arch/arm64/include/asm/elf.h | 14 ++++++++++----
 arch/arm64/kernel/process.c  | 18 ++++++------------
 2 files changed, 16 insertions(+), 16 deletions(-)

diff --git a/arch/arm64/include/asm/elf.h b/arch/arm64/include/asm/elf.h
index c8678a8c36d5..a6e9032b951a 100644
--- a/arch/arm64/include/asm/elf.h
+++ b/arch/arm64/include/asm/elf.h
@@ -253,7 +253,8 @@ struct arch_elf_state {
 	int flags;
 };
 
-#define ARM64_ELF_BTI		(1 << 0)
+#define ARM64_ELF_INTERP_BTI		(1 << 0)
+#define ARM64_ELF_EXEC_BTI		(1 << 1)
 
 #define INIT_ARCH_ELF_STATE {			\
 	.flags = 0,				\
@@ -274,9 +275,14 @@ static inline int arch_parse_elf_property(u32 type, const void *data,
 		if (datasz != sizeof(*p))
 			return -ENOEXEC;
 
-		if (system_supports_bti() && is_interp &&
-		    (*p & GNU_PROPERTY_AARCH64_FEATURE_1_BTI))
-			arch->flags |= ARM64_ELF_BTI;
+		if (system_supports_bti() &&
+		    (*p & GNU_PROPERTY_AARCH64_FEATURE_1_BTI)) {
+			if (is_interp) {
+				arch->flags |= ARM64_ELF_INTERP_BTI;
+			} else {
+				arch->flags |= ARM64_ELF_EXEC_BTI;
+			}
+		}
 	}
 
 	return 0;
diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
index b4bb67f17a2c..f7fff4a4c99f 100644
--- a/arch/arm64/kernel/process.c
+++ b/arch/arm64/kernel/process.c
@@ -744,19 +744,13 @@ asmlinkage void __sched arm64_preempt_schedule_irq(void)
 int arch_elf_adjust_prot(int prot, const struct arch_elf_state *state,
 			 bool has_interp, bool is_interp)
 {
-	/*
-	 * For dynamically linked executables the interpreter is
-	 * responsible for setting PROT_BTI on everything except
-	 * itself.
-	 */
-	if (is_interp != has_interp)
-		return prot;
+	if (prot & PROT_EXEC) {
+		if (state->flags & ARM64_ELF_INTERP_BTI && is_interp)
+			prot |= PROT_BTI;
 
-	if (!(state->flags & ARM64_ELF_BTI))
-		return prot;
-
-	if (prot & PROT_EXEC)
-		prot |= PROT_BTI;
+		if (state->flags & ARM64_ELF_EXEC_BTI && !is_interp)
+			prot |= PROT_BTI;
+	}
 
 	return prot;
 }
-- 
2.20.1