[PATCH 4.14 023/125] ARM: 8997/2: hw_breakpoint: Handle inexact watchpoint addresses

Greg Kroah-Hartman
2020-11-03T21:11:01Z

From: Douglas Anderson <dianders@chromium.org>

[ Upstream commit 22c9e58299e5f18274788ce54c03d4fb761e3c5d ]

This is commit fdfeff0f9e3d ("arm64: hw_breakpoint: Handle inexact
watchpoint addresses") but ported to arm32, which has the same
problem.

This problem was found by Android CTS tests, notably the
"watchpoint_imprecise" test [1]. I tested locally against a copycat
(simplified) version of the test though.

[1] https://android.googlesource.com/platform/bionic/+/master/tests/sys_ptrace_test.cpp

Link: https://lkml.kernel.org/r/20191019111216.1.I82eae759ca6dc28a245b043f485ca490e3015321@changeid

Signed-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Matthias Kaehlcke <mka@chromium.org>
Acked-by: Will Deacon <will@kernel.org>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
arch/arm/kernel/hw_breakpoint.c | 100 +++++++++++++++++++++++---------
1 file changed, 72 insertions(+), 28 deletions(-)

diff --git a/arch/arm/kernel/hw_breakpoint.c b/arch/arm/kernel/hw_breakpoint.c
index a30f656f791f3..e61697fb7efea 100644
--- a/arch/arm/kernel/hw_breakpoint.c
+++ b/arch/arm/kernel/hw_breakpoint.c
@@ -688,6 +688,40 @@ static void disable_single_step(struct perf_event *bp)
arch_install_hw_breakpoint(bp);
}

+/*
+ * Arm32 hardware does not always report a watchpoint hit address that matches
+ * one of the watchpoints set. It can also report an address "near" the
+ * watchpoint if a single instruction access both watched and unwatched
+ * addresses. There is no straight-forward way, short of disassembling the
+ * offending instruction, to map that address back to the watchpoint. This
+ * function computes the distance of the memory access from the watchpoint as a
+ * heuristic for the likelyhood that a given access triggered the watchpoint.
+ *
+ * See this same function in the arm64 platform code, which has the same
+ * problem.
+ *
+ * The function returns the distance of the address from the bytes watched by
+ * the watchpoint. In case of an exact match, it returns 0.
+ */
+static u32 get_distance_from_watchpoint(unsigned long addr, u32 val,
+ struct arch_hw_breakpoint_ctrl *ctrl)
+{
+ u32 wp_low, wp_high;
+ u32 lens, lene;
+
+ lens = __ffs(ctrl->len);
+ lene = __fls(ctrl->len);
+
+ wp_low = val + lens;
+ wp_high = val + lene;
+ if (addr < wp_low)
+ return wp_low - addr;
+ else if (addr > wp_high)
+ return addr - wp_high;
+ else
+ return 0;
+}
+
static int watchpoint_fault_on_uaccess(struct pt_regs *regs,
struct arch_hw_breakpoint *info)
{
@@ -697,23 +731,25 @@ static int watchpoint_fault_on_uaccess(struct pt_regs *regs,
static void watchpoint_handler(unsigned long addr, unsigned int fsr,
struct pt_regs *regs)
{
- int i, access;
- u32 val, ctrl_reg, alignment_mask;
+ int i, access, closest_match = 0;
+ u32 min_dist = -1, dist;
+ u32 val, ctrl_reg;
struct perf_event *wp, **slots;
struct arch_hw_breakpoint *info;
struct arch_hw_breakpoint_ctrl ctrl;

slots = this_cpu_ptr(wp_on_reg);

+ /*
+ * Find all watchpoints that match the reported address. If no exact
+ * match is found. Attribute the hit to the closest watchpoint.
+ */
+ rcu_read_lock();
for (i = 0; i < core_num_wrps; ++i) {
- rcu_read_lock();
-
wp = slots[i];
-
if (wp == NULL)
- goto unlock;
+ continue;

- info = counter_arch_bp(wp);
/*
* The DFAR is an unknown value on debug architectures prior
* to 7.1. Since we only allow a single watchpoint on these
@@ -722,33 +758,31 @@ static void watchpoint_handler(unsigned long addr, unsigned int fsr,
*/
if (debug_arch < ARM_DEBUG_ARCH_V7_1) {
BUG_ON(i > 0);
+ info = counter_arch_bp(wp);
info->trigger = wp->attr.bp_addr;
} else {
- if (info->ctrl.len == ARM_BREAKPOINT_LEN_8)
- alignment_mask = 0x7;
- else
- alignment_mask = 0x3;
-
- /* Check if the watchpoint value matches. */
- val = read_wb_reg(ARM_BASE_WVR + i);
- if (val != (addr & ~alignment_mask))
- goto unlock;
-
- /* Possible match, check the byte address select. */
- ctrl_reg = read_wb_reg(ARM_BASE_WCR + i);
- decode_ctrl_reg(ctrl_reg, &ctrl);
- if (!((1 << (addr & alignment_mask)) & ctrl.len))
- goto unlock;
-
/* Check that the access type matches. */
if (debug_exception_updates_fsr()) {
access = (fsr & ARM_FSR_ACCESS_MASK) ?
HW_BREAKPOINT_W : HW_BREAKPOINT_R;
if (!(access & hw_breakpoint_type(wp)))
- goto unlock;
+ continue;
}

+ val = read_wb_reg(ARM_BASE_WVR + i);
+ ctrl_reg = read_wb_reg(ARM_BASE_WCR + i);
+ decode_ctrl_reg(ctrl_reg, &ctrl);
+ dist = get_distance_from_watchpoint(addr, val, &ctrl);
+ if (dist < min_dist) {
+ min_dist = dist;
+ closest_match = i;
+ }
+ /* Is this an exact match? */
+ if (dist != 0)
+ continue;
+
/* We have a winner. */
+ info = counter_arch_bp(wp);
info->trigger = addr;
}

@@ -770,13 +804,23 @@ static void watchpoint_handler(unsigned long addr, unsigned int fsr,
* we can single-step over the watchpoint trigger.
*/
if (!is_default_overflow_handler(wp))
- goto unlock;
-
+ continue;
step:
enable_single_step(wp, instruction_pointer(regs));
-unlock:
- rcu_read_unlock();
}
+
+ if (min_dist > 0 && min_dist != -1) {
+ /* No exact match found. */
+ wp = slots[closest_match];
+ info = counter_arch_bp(wp);
+ info->trigger = addr;
+ pr_debug("watchpoint fired: address = 0x%x\n", info->trigger);
+ perf_bp_event(wp, regs);
+ if (is_default_overflow_handler(wp))
+ enable_single_step(wp, instruction_pointer(regs));
+ }
+
+ rcu_read_unlock();
}

static void watchpoint_single_step_handler(unsigned long pc)
--
2.27.0