[PATCH] kfence: Use pt_regs to generate stack trace on faults

From: Marco Elver
Date: Thu Nov 05 2020 - 04:21:48 EST

Instead of removing the fault handling portion of the stack trace based
on the fault handler's name, just use struct pt_regs directly.

Change kfence_handle_page_fault() to take a struct pt_regs, and plumb it
through to kfence_report_error() for out-of-bounds, use-after-free, or
invalid access errors, where pt_regs is used to generate the stack
trace.

If the kernel is a DEBUG_KERNEL, also show registers for more
information.

Suggested-by: Mark Rutland <mark.rutland@xxxxxxx>
Signed-off-by: Marco Elver <elver@xxxxxxxxxx>
---
arch/arm64/include/asm/kfence.h | 2 --
arch/arm64/mm/fault.c | 2 +-
arch/x86/include/asm/kfence.h | 6 ----
arch/x86/mm/fault.c | 2 +-
include/linux/kfence.h | 5 +--
mm/kfence/core.c | 10 +++---
mm/kfence/kfence.h | 4 +--
mm/kfence/report.c | 63 +++++++++++++++++++--------------
8 files changed, 48 insertions(+), 46 deletions(-)

diff --git a/arch/arm64/include/asm/kfence.h b/arch/arm64/include/asm/kfence.h
index 5ac0f599cc9a..6c0afeeab635 100644
--- a/arch/arm64/include/asm/kfence.h
+++ b/arch/arm64/include/asm/kfence.h
@@ -5,8 +5,6 @@

#include <asm/cacheflush.h>

-#define KFENCE_SKIP_ARCH_FAULT_HANDLER "el1_sync"
-
static inline bool arch_kfence_init_pool(void) { return true; }

static inline bool kfence_protect_page(unsigned long addr, bool protect)
diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
index 2d60204b4ed2..183d1e6dd9e0 100644
--- a/arch/arm64/mm/fault.c
+++ b/arch/arm64/mm/fault.c
@@ -323,7 +323,7 @@ static void __do_kernel_fault(unsigned long addr, unsigned int esr,
} else if (addr < PAGE_SIZE) {
msg = "NULL pointer dereference";
} else {
- if (kfence_handle_page_fault(addr))
+ if (kfence_handle_page_fault(addr, regs))
return;

msg = "paging request";
diff --git a/arch/x86/include/asm/kfence.h b/arch/x86/include/asm/kfence.h
index beeac105dae7..2f3f877a7a5c 100644
--- a/arch/x86/include/asm/kfence.h
+++ b/arch/x86/include/asm/kfence.h
@@ -11,12 +11,6 @@
#include <asm/set_memory.h>
#include <asm/tlbflush.h>

-/*
- * The page fault handler entry function, up to which the stack trace is
- * truncated in reports.
- */
-#define KFENCE_SKIP_ARCH_FAULT_HANDLER "asm_exc_page_fault"
-
/* Force 4K pages for __kfence_pool. */
static inline bool arch_kfence_init_pool(void)
{
diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index e42db2836438..53d732161b4f 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -727,7 +727,7 @@ no_context(struct pt_regs *regs, unsigned long error_code,
efi_recover_from_page_fault(address);

/* Only not-present faults should be handled by KFENCE. */
- if (!(error_code & X86_PF_PROT) && kfence_handle_page_fault(address))
+ if (!(error_code & X86_PF_PROT) && kfence_handle_page_fault(address, regs))
return;

oops:
diff --git a/include/linux/kfence.h b/include/linux/kfence.h
index ed2d48acdafe..98a97f9d43cd 100644
--- a/include/linux/kfence.h
+++ b/include/linux/kfence.h
@@ -171,6 +171,7 @@ static __always_inline __must_check bool kfence_free(void *addr)
/**
* kfence_handle_page_fault() - perform page fault handling for KFENCE pages
* @addr: faulting address
+ * @regs: current struct pt_regs (can be NULL, but shows full stack trace)
*
* Return:
* * false - address outside KFENCE pool,
@@ -181,7 +182,7 @@ static __always_inline __must_check bool kfence_free(void *addr)
* cases KFENCE prints an error message and marks the offending page as
* present, so that the kernel can proceed.
*/
-bool __must_check kfence_handle_page_fault(unsigned long addr);
+bool __must_check kfence_handle_page_fault(unsigned long addr, struct pt_regs *regs);

#else /* CONFIG_KFENCE */

@@ -194,7 +195,7 @@ static inline size_t kfence_ksize(const void *addr) { return 0; }
static inline void *kfence_object_start(const void *addr) { return NULL; }
static inline void __kfence_free(void *addr) { }
static inline bool __must_check kfence_free(void *addr) { return false; }
-static inline bool __must_check kfence_handle_page_fault(unsigned long addr) { return false; }
+static inline bool __must_check kfence_handle_page_fault(unsigned long addr, struct pt_regs *regs) { return false; }

#endif

diff --git a/mm/kfence/core.c b/mm/kfence/core.c
index 9d597013cd5d..9358f42a9a9e 100644
--- a/mm/kfence/core.c
+++ b/mm/kfence/core.c
@@ -212,7 +212,7 @@ static inline bool check_canary_byte(u8 *addr)
return true;

atomic_long_inc(&counters[KFENCE_COUNTER_BUGS]);
- kfence_report_error((unsigned long)addr, addr_to_metadata((unsigned long)addr),
+ kfence_report_error((unsigned long)addr, NULL, addr_to_metadata((unsigned long)addr),
KFENCE_ERROR_CORRUPTION);
return false;
}
@@ -351,7 +351,7 @@ static void kfence_guarded_free(void *addr, struct kfence_metadata *meta, bool z
if (meta->state != KFENCE_OBJECT_ALLOCATED || meta->addr != (unsigned long)addr) {
/* Invalid or double-free, bail out. */
atomic_long_inc(&counters[KFENCE_COUNTER_BUGS]);
- kfence_report_error((unsigned long)addr, meta, KFENCE_ERROR_INVALID_FREE);
+ kfence_report_error((unsigned long)addr, NULL, meta, KFENCE_ERROR_INVALID_FREE);
raw_spin_unlock_irqrestore(&meta->lock, flags);
return;
}
@@ -752,7 +752,7 @@ void __kfence_free(void *addr)
kfence_guarded_free(addr, meta, false);
}

-bool kfence_handle_page_fault(unsigned long addr)
+bool kfence_handle_page_fault(unsigned long addr, struct pt_regs *regs)
{
const int page_index = (addr - (unsigned long)__kfence_pool) / PAGE_SIZE;
struct kfence_metadata *to_report = NULL;
@@ -815,11 +815,11 @@ bool kfence_handle_page_fault(unsigned long addr)

out:
if (to_report) {
- kfence_report_error(addr, to_report, error_type);
+ kfence_report_error(addr, regs, to_report, error_type);
raw_spin_unlock_irqrestore(&to_report->lock, flags);
} else {
/* This may be a UAF or OOB access, but we can't be sure. */
- kfence_report_error(addr, NULL, KFENCE_ERROR_INVALID);
+ kfence_report_error(addr, regs, NULL, KFENCE_ERROR_INVALID);
}

return kfence_unprotect(addr); /* Unprotect and let access proceed. */
diff --git a/mm/kfence/kfence.h b/mm/kfence/kfence.h
index f115aabc2052..fa3579d03089 100644
--- a/mm/kfence/kfence.h
+++ b/mm/kfence/kfence.h
@@ -99,8 +99,8 @@ enum kfence_error_type {
KFENCE_ERROR_INVALID_FREE, /* Invalid free. */
};

-void kfence_report_error(unsigned long address, const struct kfence_metadata *meta,
- enum kfence_error_type type);
+void kfence_report_error(unsigned long address, struct pt_regs *regs,
+ const struct kfence_metadata *meta, enum kfence_error_type type);

void kfence_print_object(struct seq_file *seq, const struct kfence_metadata *meta);

diff --git a/mm/kfence/report.c b/mm/kfence/report.c
index 0fdaa3ddf1b4..4dedc2ff8f28 100644
--- a/mm/kfence/report.c
+++ b/mm/kfence/report.c
@@ -5,6 +5,7 @@
#include <linux/kernel.h>
#include <linux/lockdep.h>
#include <linux/printk.h>
+#include <linux/sched/debug.h>
#include <linux/seq_file.h>
#include <linux/stacktrace.h>
#include <linux/string.h>
@@ -36,7 +37,6 @@ static int get_stack_skipnr(const unsigned long stack_entries[], int num_entries
{
char buf[64];
int skipnr, fallback = 0;
- bool is_access_fault = false;

if (type) {
/* Depending on error type, find different stack entries. */
@@ -44,8 +44,12 @@ static int get_stack_skipnr(const unsigned long stack_entries[], int num_entries
case KFENCE_ERROR_UAF:
case KFENCE_ERROR_OOB:
case KFENCE_ERROR_INVALID:
- is_access_fault = true;
- break;
+ /*
+ * kfence_handle_page_fault() may be called with pt_regs
+ * set to NULL; in that case we'll simply show the full
+ * stack trace.
+ */
+ return 0;
case KFENCE_ERROR_CORRUPTION:
case KFENCE_ERROR_INVALID_FREE:
break;
@@ -55,26 +59,21 @@ static int get_stack_skipnr(const unsigned long stack_entries[], int num_entries
for (skipnr = 0; skipnr < num_entries; skipnr++) {
int len = scnprintf(buf, sizeof(buf), "%ps", (void *)stack_entries[skipnr]);

- if (is_access_fault) {
- if (!strncmp(buf, KFENCE_SKIP_ARCH_FAULT_HANDLER, len))
- goto found;
- } else {
- if (str_has_prefix(buf, "kfence_") || str_has_prefix(buf, "__kfence_") ||
- !strncmp(buf, "__slab_free", len)) {
- /*
- * In case of tail calls from any of the below
- * to any of the above.
- */
- fallback = skipnr + 1;
- }
-
- /* Also the *_bulk() variants by only checking prefixes. */
- if (str_has_prefix(buf, "kfree") ||
- str_has_prefix(buf, "kmem_cache_free") ||
- str_has_prefix(buf, "__kmalloc") ||
- str_has_prefix(buf, "kmem_cache_alloc"))
- goto found;
+ if (str_has_prefix(buf, "kfence_") || str_has_prefix(buf, "__kfence_") ||
+ !strncmp(buf, "__slab_free", len)) {
+ /*
+ * In case of tail calls from any of the below
+ * to any of the above.
+ */
+ fallback = skipnr + 1;
}
+
+ /* Also the *_bulk() variants by only checking prefixes. */
+ if (str_has_prefix(buf, "kfree") ||
+ str_has_prefix(buf, "kmem_cache_free") ||
+ str_has_prefix(buf, "__kmalloc") ||
+ str_has_prefix(buf, "kmem_cache_alloc"))
+ goto found;
}
if (fallback < num_entries)
return fallback;
@@ -152,13 +151,20 @@ static void print_diff_canary(unsigned long address, size_t bytes_to_show,
pr_cont(" ]");
}

-void kfence_report_error(unsigned long address, const struct kfence_metadata *meta,
- enum kfence_error_type type)
+void kfence_report_error(unsigned long address, struct pt_regs *regs,
+ const struct kfence_metadata *meta, enum kfence_error_type type)
{
unsigned long stack_entries[KFENCE_STACK_DEPTH] = { 0 };
- int num_stack_entries = stack_trace_save(stack_entries, KFENCE_STACK_DEPTH, 1);
- int skipnr = get_stack_skipnr(stack_entries, num_stack_entries, &type);
const ptrdiff_t object_index = meta ? meta - kfence_metadata : -1;
+ int num_stack_entries;
+ int skipnr = 0;
+
+ if (regs) {
+ num_stack_entries = stack_trace_save_regs(regs, stack_entries, KFENCE_STACK_DEPTH, 0);
+ } else {
+ num_stack_entries = stack_trace_save(stack_entries, KFENCE_STACK_DEPTH, 1);
+ skipnr = get_stack_skipnr(stack_entries, num_stack_entries, &type);
+ }

/* Require non-NULL meta, except if KFENCE_ERROR_INVALID. */
if (WARN_ON(type != KFENCE_ERROR_INVALID && !meta))
@@ -222,7 +228,10 @@ void kfence_report_error(unsigned long address, const struct kfence_metadata *me

/* Print report footer. */
pr_err("\n");
- dump_stack_print_info(KERN_ERR);
+ if (IS_ENABLED(CONFIG_DEBUG_KERNEL) && regs)
+ show_regs(regs);
+ else
+ dump_stack_print_info(KERN_ERR);
pr_err("==================================================================\n");

lockdep_on();
--
2.29.1.341.ge80a0c044ae-goog