Re: [PATCH] kfence: show cpu and timestamp in alloc/free info

From: Alexander Potapenko
Date: Tue Jun 29 2021 - 07:35:08 EST

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On Tue, Jun 29, 2021 at 1:33 PM Marco Elver <elver@xxxxxxxxxx> wrote:
>
> Record cpu and timestamp on allocations and frees, and show them in
> reports. Upon an error, this can help correlate earlier messages in the
> kernel log via allocation and free timestamps.
>
> Suggested-by: Joern Engel <joern@xxxxxxxxxxxxxxx>
> Signed-off-by: Marco Elver <elver@xxxxxxxxxx>

Acked-by: Alexander Potapenko <glider@xxxxxxxxxx>

Thanks!

> ---
> Documentation/dev-tools/kfence.rst | 98 ++++++++++++++++--------------
> mm/kfence/core.c | 3 +
> mm/kfence/kfence.h | 2 +
> mm/kfence/report.c | 19 ++++--
> 4 files changed, 71 insertions(+), 51 deletions(-)
>
> diff --git a/Documentation/dev-tools/kfence.rst b/Documentation/dev-tools/kfence.rst
> index fdf04e741ea5..0fbe3308bf37 100644
> --- a/Documentation/dev-tools/kfence.rst
> +++ b/Documentation/dev-tools/kfence.rst
> @@ -65,25 +65,27 @@ Error reports
> A typical out-of-bounds access looks like this::
>
> ==================================================================
> - BUG: KFENCE: out-of-bounds read in test_out_of_bounds_read+0xa3/0x22b
> + BUG: KFENCE: out-of-bounds read in test_out_of_bounds_read+0xa6/0x234
>
> - Out-of-bounds read at 0xffffffffb672efff (1B left of kfence-#17):
> - test_out_of_bounds_read+0xa3/0x22b
> - kunit_try_run_case+0x51/0x85
> + Out-of-bounds read at 0xffff8c3f2e291fff (1B left of kfence-#72):
> + test_out_of_bounds_read+0xa6/0x234
> + kunit_try_run_case+0x61/0xa0
> kunit_generic_run_threadfn_adapter+0x16/0x30
> - kthread+0x137/0x160
> + kthread+0x176/0x1b0
> ret_from_fork+0x22/0x30
>
> - kfence-#17 [0xffffffffb672f000-0xffffffffb672f01f, size=32, cache=kmalloc-32] allocated by task 507:
> - test_alloc+0xf3/0x25b
> - test_out_of_bounds_read+0x98/0x22b
> - kunit_try_run_case+0x51/0x85
> + kfence-#72: 0xffff8c3f2e292000-0xffff8c3f2e29201f, size=32, cache=kmalloc-32
> +
> + allocated by task 484 on cpu 0 at 32.919330s:
> + test_alloc+0xfe/0x738
> + test_out_of_bounds_read+0x9b/0x234
> + kunit_try_run_case+0x61/0xa0
> kunit_generic_run_threadfn_adapter+0x16/0x30
> - kthread+0x137/0x160
> + kthread+0x176/0x1b0
> ret_from_fork+0x22/0x30
>
> - CPU: 4 PID: 107 Comm: kunit_try_catch Not tainted 5.8.0-rc6+ #7
> - Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-1 04/01/2014
> + CPU: 0 PID: 484 Comm: kunit_try_catch Not tainted 5.13.0-rc3+ #7
> + Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.14.0-2 04/01/2014
> ==================================================================
>
> The header of the report provides a short summary of the function involved in
> @@ -96,30 +98,32 @@ Use-after-free accesses are reported as::
> ==================================================================
> BUG: KFENCE: use-after-free read in test_use_after_free_read+0xb3/0x143
>
> - Use-after-free read at 0xffffffffb673dfe0 (in kfence-#24):
> + Use-after-free read at 0xffff8c3f2e2a0000 (in kfence-#79):
> test_use_after_free_read+0xb3/0x143
> - kunit_try_run_case+0x51/0x85
> + kunit_try_run_case+0x61/0xa0
> kunit_generic_run_threadfn_adapter+0x16/0x30
> - kthread+0x137/0x160
> + kthread+0x176/0x1b0
> ret_from_fork+0x22/0x30
>
> - kfence-#24 [0xffffffffb673dfe0-0xffffffffb673dfff, size=32, cache=kmalloc-32] allocated by task 507:
> - test_alloc+0xf3/0x25b
> + kfence-#79: 0xffff8c3f2e2a0000-0xffff8c3f2e2a001f, size=32, cache=kmalloc-32
> +
> + allocated by task 488 on cpu 2 at 33.871326s:
> + test_alloc+0xfe/0x738
> test_use_after_free_read+0x76/0x143
> - kunit_try_run_case+0x51/0x85
> + kunit_try_run_case+0x61/0xa0
> kunit_generic_run_threadfn_adapter+0x16/0x30
> - kthread+0x137/0x160
> + kthread+0x176/0x1b0
> ret_from_fork+0x22/0x30
>
> - freed by task 507:
> + freed by task 488 on cpu 2 at 33.871358s:
> test_use_after_free_read+0xa8/0x143
> - kunit_try_run_case+0x51/0x85
> + kunit_try_run_case+0x61/0xa0
> kunit_generic_run_threadfn_adapter+0x16/0x30
> - kthread+0x137/0x160
> + kthread+0x176/0x1b0
> ret_from_fork+0x22/0x30
>
> - CPU: 4 PID: 109 Comm: kunit_try_catch Tainted: G W 5.8.0-rc6+ #7
> - Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-1 04/01/2014
> + CPU: 2 PID: 488 Comm: kunit_try_catch Tainted: G B 5.13.0-rc3+ #7
> + Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.14.0-2 04/01/2014
> ==================================================================
>
> KFENCE also reports on invalid frees, such as double-frees::
> @@ -127,30 +131,32 @@ KFENCE also reports on invalid frees, such as double-frees::
> ==================================================================
> BUG: KFENCE: invalid free in test_double_free+0xdc/0x171
>
> - Invalid free of 0xffffffffb6741000:
> + Invalid free of 0xffff8c3f2e2a4000 (in kfence-#81):
> test_double_free+0xdc/0x171
> - kunit_try_run_case+0x51/0x85
> + kunit_try_run_case+0x61/0xa0
> kunit_generic_run_threadfn_adapter+0x16/0x30
> - kthread+0x137/0x160
> + kthread+0x176/0x1b0
> ret_from_fork+0x22/0x30
>
> - kfence-#26 [0xffffffffb6741000-0xffffffffb674101f, size=32, cache=kmalloc-32] allocated by task 507:
> - test_alloc+0xf3/0x25b
> + kfence-#81: 0xffff8c3f2e2a4000-0xffff8c3f2e2a401f, size=32, cache=kmalloc-32
> +
> + allocated by task 490 on cpu 1 at 34.175321s:
> + test_alloc+0xfe/0x738
> test_double_free+0x76/0x171
> - kunit_try_run_case+0x51/0x85
> + kunit_try_run_case+0x61/0xa0
> kunit_generic_run_threadfn_adapter+0x16/0x30
> - kthread+0x137/0x160
> + kthread+0x176/0x1b0
> ret_from_fork+0x22/0x30
>
> - freed by task 507:
> + freed by task 490 on cpu 1 at 34.175348s:
> test_double_free+0xa8/0x171
> - kunit_try_run_case+0x51/0x85
> + kunit_try_run_case+0x61/0xa0
> kunit_generic_run_threadfn_adapter+0x16/0x30
> - kthread+0x137/0x160
> + kthread+0x176/0x1b0
> ret_from_fork+0x22/0x30
>
> - CPU: 4 PID: 111 Comm: kunit_try_catch Tainted: G W 5.8.0-rc6+ #7
> - Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-1 04/01/2014
> + CPU: 1 PID: 490 Comm: kunit_try_catch Tainted: G B 5.13.0-rc3+ #7
> + Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.14.0-2 04/01/2014
> ==================================================================
>
> KFENCE also uses pattern-based redzones on the other side of an object's guard
> @@ -160,23 +166,25 @@ These are reported on frees::
> ==================================================================
> BUG: KFENCE: memory corruption in test_kmalloc_aligned_oob_write+0xef/0x184
>
> - Corrupted memory at 0xffffffffb6797ff9 [ 0xac . . . . . . ] (in kfence-#69):
> + Corrupted memory at 0xffff8c3f2e33aff9 [ 0xac . . . . . . ] (in kfence-#156):
> test_kmalloc_aligned_oob_write+0xef/0x184
> - kunit_try_run_case+0x51/0x85
> + kunit_try_run_case+0x61/0xa0
> kunit_generic_run_threadfn_adapter+0x16/0x30
> - kthread+0x137/0x160
> + kthread+0x176/0x1b0
> ret_from_fork+0x22/0x30
>
> - kfence-#69 [0xffffffffb6797fb0-0xffffffffb6797ff8, size=73, cache=kmalloc-96] allocated by task 507:
> - test_alloc+0xf3/0x25b
> + kfence-#156: 0xffff8c3f2e33afb0-0xffff8c3f2e33aff8, size=73, cache=kmalloc-96
> +
> + allocated by task 502 on cpu 7 at 42.159302s:
> + test_alloc+0xfe/0x738
> test_kmalloc_aligned_oob_write+0x57/0x184
> - kunit_try_run_case+0x51/0x85
> + kunit_try_run_case+0x61/0xa0
> kunit_generic_run_threadfn_adapter+0x16/0x30
> - kthread+0x137/0x160
> + kthread+0x176/0x1b0
> ret_from_fork+0x22/0x30
>
> - CPU: 4 PID: 120 Comm: kunit_try_catch Tainted: G W 5.8.0-rc6+ #7
> - Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-1 04/01/2014
> + CPU: 7 PID: 502 Comm: kunit_try_catch Tainted: G B 5.13.0-rc3+ #7
> + Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.14.0-2 04/01/2014
> ==================================================================
>
> For such errors, the address where the corruption occurred as well as the
> diff --git a/mm/kfence/core.c b/mm/kfence/core.c
> index d7666ace9d2e..0fd7a122e1a1 100644
> --- a/mm/kfence/core.c
> +++ b/mm/kfence/core.c
> @@ -20,6 +20,7 @@
> #include <linux/moduleparam.h>
> #include <linux/random.h>
> #include <linux/rcupdate.h>
> +#include <linux/sched/clock.h>
> #include <linux/sched/sysctl.h>
> #include <linux/seq_file.h>
> #include <linux/slab.h>
> @@ -196,6 +197,8 @@ static noinline void metadata_update_state(struct kfence_metadata *meta,
> */
> track->num_stack_entries = stack_trace_save(track->stack_entries, KFENCE_STACK_DEPTH, 1);
> track->pid = task_pid_nr(current);
> + track->cpu = raw_smp_processor_id();
> + track->ts_nsec = local_clock(); /* Same source as printk timestamps. */
>
> /*
> * Pairs with READ_ONCE() in
> diff --git a/mm/kfence/kfence.h b/mm/kfence/kfence.h
> index 24065321ff8a..c1f23c61e5f9 100644
> --- a/mm/kfence/kfence.h
> +++ b/mm/kfence/kfence.h
> @@ -36,6 +36,8 @@ enum kfence_object_state {
> /* Alloc/free tracking information. */
> struct kfence_track {
> pid_t pid;
> + int cpu;
> + u64 ts_nsec;
> int num_stack_entries;
> unsigned long stack_entries[KFENCE_STACK_DEPTH];
> };
> diff --git a/mm/kfence/report.c b/mm/kfence/report.c
> index 2a319c21c939..d1daabdc9188 100644
> --- a/mm/kfence/report.c
> +++ b/mm/kfence/report.c
> @@ -9,6 +9,7 @@
>
> #include <linux/kernel.h>
> #include <linux/lockdep.h>
> +#include <linux/math.h>
> #include <linux/printk.h>
> #include <linux/sched/debug.h>
> #include <linux/seq_file.h>
> @@ -100,6 +101,13 @@ static void kfence_print_stack(struct seq_file *seq, const struct kfence_metadat
> bool show_alloc)
> {
> const struct kfence_track *track = show_alloc ? &meta->alloc_track : &meta->free_track;
> + u64 ts_sec = track->ts_nsec;
> + unsigned long rem_nsec = do_div(ts_sec, NSEC_PER_SEC);
> +
> + /* Timestamp matches printk timestamp format. */
> + seq_con_printf(seq, "%s by task %d on cpu %d at %lu.%06lus:\n",
> + show_alloc ? "allocated" : "freed", meta->alloc_track.pid,
> + meta->alloc_track.cpu, (unsigned long)ts_sec, rem_nsec / 1000);
>
> if (track->num_stack_entries) {
> /* Skip allocation/free internals stack. */
> @@ -126,15 +134,14 @@ void kfence_print_object(struct seq_file *seq, const struct kfence_metadata *met
> return;
> }
>
> - seq_con_printf(seq,
> - "kfence-#%td [0x%p-0x%p"
> - ", size=%d, cache=%s] allocated by task %d:\n",
> - meta - kfence_metadata, (void *)start, (void *)(start + size - 1), size,
> - (cache && cache->name) ? cache->name : "<destroyed>", meta->alloc_track.pid);
> + seq_con_printf(seq, "kfence-#%td: 0x%p-0x%p, size=%d, cache=%s\n\n",
> + meta - kfence_metadata, (void *)start, (void *)(start + size - 1),
> + size, (cache && cache->name) ? cache->name : "<destroyed>");
> +
> kfence_print_stack(seq, meta, true);
>
> if (meta->state == KFENCE_OBJECT_FREED) {
> - seq_con_printf(seq, "\nfreed by task %d:\n", meta->free_track.pid);
> + seq_con_printf(seq, "\n");
> kfence_print_stack(seq, meta, false);
> }
> }
> --
> 2.32.0.93.g670b81a890-goog
>

--
Alexander Potapenko
Software Engineer

Google Germany GmbH
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80636 München

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