linux-kernel - [PATCH v2 2/3] arm64: ptrace: minimize default bp

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Message-Id: <20250827014113.5852-3-b10902118@ntu.edu.tw>
Date: Wed, 27 Aug 2025 09:41:12 +0800
From: Bill Tsui <b10902118@....edu.tw>
To: oleg@...hat.com,
	linux@...linux.org.uk,
	catalin.marinas@....com,
	will@...nel.org
Cc: linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org,
	Bill Tsui <b10902118@....edu.tw>
Subject: [PATCH v2 2/3] arm64: ptrace: minimize default bp_len for hw breakpoints to pass check

PTRACE_SETHBPREGS fails when setting a hardware breakpoint on a
non-4-byte aligned address with a valid length to a 32-bit tracee. The
length should be valid as long as the range started from the address
is within one aligned 4 bytes.

The cause is that compat_ptrace_hbp_set() can only modify either
addr or ctrl of a breakpoint per call, but always checks alignment in
hw_breakpoint_arch_parse(). If a breakpoint has ctrl.len=4 (the
default), then users cannot set the addr to a non-4-byte aligned
address. Likewise, if the addr was previously set unaligned, users
cannot set ctrl.len to 4.

This patch mitigates the issue by minimizing the default bp_len, so
that any possibly valid address can pass the check with it. However, it
does not solve misaligned addr/len in modifying existing breakpoints;
further work may be needed to remove or relax the alignment check.

Signed-off-by: Bill Tsui <b10902118@....edu.tw>
---
 arch/arm64/kernel/ptrace.c | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
index 73c67f743..70c9acd94 100644
--- a/arch/arm64/kernel/ptrace.c
+++ b/arch/arm64/kernel/ptrace.c
@@ -288,14 +288,25 @@ static struct perf_event *ptrace_hbp_create(unsigned int note_type,
 {
 	struct perf_event *bp;
 	struct perf_event_attr attr;
-	int err, type;
+	int err, type, min_len;

+	/*
+	 * min_len ensures any possibly valid address can pass alignment
+	 * validation with it.
+	 * This is for compat mode, in which addr and ctrl can only be set
+	 * one after one with SETHBPREGS. If addr is set first, ctrl will
+	 * remain as initial value.
+	 */
 	switch (note_type) {
 	case NT_ARM_HW_BREAK:
 		type = HW_BREAKPOINT_X;
+		min_len = (tsk && is_compat_thread(task_thread_info(tsk))) ?
+				  HW_BREAKPOINT_LEN_2 :
+				  HW_BREAKPOINT_LEN_4;
 		break;
 	case NT_ARM_HW_WATCH:
 		type = HW_BREAKPOINT_RW;
+		min_len = HW_BREAKPOINT_LEN_1;
 		break;
 	default:
 		return ERR_PTR(-EINVAL);
@@ -308,7 +319,7 @@ static struct perf_event *ptrace_hbp_create(unsigned int note_type,
 	 * (i.e. values that will pass validation).
 	 */
 	attr.bp_addr	= 0;
-	attr.bp_len	= HW_BREAKPOINT_LEN_4;
+	attr.bp_len	= min_len;
 	attr.bp_type	= type;
 	attr.disabled	= 1;

-- 
2.50.1