[PATCH] arm64/hw_breakpoint: reject unaligned watchpoints that would truncate BAS

[Breno Leitao]

hw_breakpoint_arch_parse() positions the BAS bit pattern in
hw->ctrl.len with

	offset = hw->address & alignment_mask;	/* 0..7 */
	hw->ctrl.len <<= offset;

ctrl.len is an 8-bit bitfield (struct arch_hw_breakpoint_ctrl::len is
u32 :8), so the shift silently drops any bits past bit 7.  For
non-compat AArch64 watchpoints the offset is unbounded relative to
ctrl.len: a perf_event_open(PERF_TYPE_BREAKPOINT) caller asking for
HW_BREAKPOINT_W with bp_addr=page+1 and bp_len=HW_BREAKPOINT_LEN_8
ends up with 0xff << 1 = 0x1fe, stored as 0xfe.  The kernel programs
WCR.BAS=0xfe and the hardware watches bytes [1..7] instead of the
requested [1..8] -- the eighth byte is silently dropped.  The
syscall still returns success, leaving userspace to discover the
gap by empirical probing.

The same class affects HW_BREAKPOINT_LEN_{2,4} when offset pushes the
high BAS bit past bit 7 (e.g. LEN_4 with offset=5 yields 0xe0
instead of 0x1e0).  No memory-safety impact -- the value is masked
into 8 bits before encoding -- but debuggers and perf users observe
missed events on bytes they thought they were watching.

The AArch32 branch immediately above already rejects unrepresentable
(offset, len) combinations via an explicit switch.  Mirror that for
the non-compat branch by checking that the shifted pattern fits in
the BAS field, returning -EINVAL when it does not.

Reproducer:

  struct perf_event_attr a = {
      .type = PERF_TYPE_BREAKPOINT, .size = sizeof(a),
      .bp_type = HW_BREAKPOINT_W,
      .bp_addr = (uintptr_t)(buf + 1),
      .bp_len = HW_BREAKPOINT_LEN_8,
      .exclude_kernel = 1, .exclude_hv = 1,
  };
  int fd = perf_event_open(&a, 0, -1, -1, 0);
  /* before this fix: succeeds, watches 7 bytes (buf+1..buf+7)   */
  /* after  this fix: fails with EINVAL                          */

Signed-off-by: Breno Leitao <leitao at debian.org>
Fixes: b08fb180bb88 ("arm64: Allow hw watchpoint at varied offset from base address")
---
 arch/arm64/kernel/hw_breakpoint.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/arch/arm64/kernel/hw_breakpoint.c b/arch/arm64/kernel/hw_breakpoint.c
index ab76b36dce820..b8a1402119f3a 100644
--- a/arch/arm64/kernel/hw_breakpoint.c
+++ b/arch/arm64/kernel/hw_breakpoint.c
@@ -559,6 +559,15 @@ int hw_breakpoint_arch_parse(struct perf_event *bp,
 		else
 			alignment_mask = 0x7;
 		offset = hw->address & alignment_mask;
+
+		/*
+		 * BAS is an 8-bit field in WCR/BCR; the shift below would
+		 * silently drop the high bits of ctrl.len when offset + len
+		 * exceeds 8, programming hardware to watch fewer bytes than
+		 * the user requested.
+		 */
+		if (((u32)hw->ctrl.len << offset) > 0xff)
+			return -EINVAL;
 	}
 
 	hw->address &= ~alignment_mask;

---
base-commit: 0787c45ea08a13b5482e701fabc741877cf681f6
change-id: 20260430-arm64_bas-77e37d830226

Best regards,
--  
Breno Leitao <leitao at debian.org>