This reverts commit 487a32ec24be819e747af8c2ab0d5c515508086a.
The should_skip_kasan_poison() function reads the PG_skip_kasan_poison
flag from page->flags. However, this line of code in free_pages_prepare():
page->flags &= ~PAGE_FLAGS_CHECK_AT_PREP;
clears most of page->flags, including PG_skip_kasan_poison, before calling
should_skip_kasan_poison(), which meant that it would never return true
as a result of the page flag being set. Therefore, fix the code to call
should_skip_kasan_poison() before clearing the flags, as we were doing
before the reverted patch.
This fixes a measurable performance regression introduced in the
reverted commit, where munmap() takes longer than intended if HW
tags KASAN is supported and enabled at runtime. Without this patch,
we see a single-digit percentage performance regression in a particular
mmap()-heavy benchmark when enabling HW tags KASAN, and with the patch,
there is no statistically significant performance impact when enabling
HW tags KASAN.
Signed-off-by: Peter Collingbourne <pcc(a)google.com>
Fixes: 487a32ec24be ("kasan: drop skip_kasan_poison variable in free_pages_prepare")
Cc: <stable(a)vger.kernel.org> # 6.1
Link: https://linux-review.googlesource.com/id/Ic4f13affeebd20548758438bb9ed9ca40…
Reviewed-by: Andrey Konovalov <andreyknvl(a)gmail.com>
---
mm/page_alloc.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 1c54790c2d17..c58ebf21ce63 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1413,6 +1413,7 @@ static __always_inline bool free_pages_prepare(struct page *page,
unsigned int order, fpi_t fpi_flags)
{
int bad = 0;
+ bool skip_kasan_poison = should_skip_kasan_poison(page, fpi_flags);
bool init = want_init_on_free();
VM_BUG_ON_PAGE(PageTail(page), page);
@@ -1489,7 +1490,7 @@ static __always_inline bool free_pages_prepare(struct page *page,
* With hardware tag-based KASAN, memory tags must be set before the
* page becomes unavailable via debug_pagealloc or arch_free_page.
*/
- if (!should_skip_kasan_poison(page, fpi_flags)) {
+ if (!skip_kasan_poison) {
kasan_poison_pages(page, order, init);
/* Memory is already initialized if KASAN did it internally. */
--
2.40.0.rc1.284.g88254d51c5-goog
Haibo Li reported:
| Unable to handle kernel paging request at virtual address
| ffffff802a0d8d7171
| Mem abort info:o:
| ESR = 0x9600002121
| EC = 0x25: DABT (current EL), IL = 32 bitsts
| SET = 0, FnV = 0 0
| EA = 0, S1PTW = 0 0
| FSC = 0x21: alignment fault
| Data abort info:o:
| ISV = 0, ISS = 0x0000002121
| CM = 0, WnR = 0 0
| swapper pgtable: 4k pages, 39-bit VAs, pgdp=000000002835200000
| [ffffff802a0d8d71] pgd=180000005fbf9003, p4d=180000005fbf9003,
| pud=180000005fbf9003, pmd=180000005fbe8003, pte=006800002a0d8707
| Internal error: Oops: 96000021 [#1] PREEMPT SMP
| Modules linked in:
| CPU: 2 PID: 45 Comm: kworker/u8:2 Not tainted
| 5.15.78-android13-8-g63561175bbda-dirty #1
| ...
| pc : kcsan_setup_watchpoint+0x26c/0x6bc
| lr : kcsan_setup_watchpoint+0x88/0x6bc
| sp : ffffffc00ab4b7f0
| x29: ffffffc00ab4b800 x28: ffffff80294fe588 x27: 0000000000000001
| x26: 0000000000000019 x25: 0000000000000001 x24: ffffff80294fdb80
| x23: 0000000000000000 x22: ffffffc00a70fb68 x21: ffffff802a0d8d71
| x20: 0000000000000002 x19: 0000000000000000 x18: ffffffc00a9bd060
| x17: 0000000000000001 x16: 0000000000000000 x15: ffffffc00a59f000
| x14: 0000000000000001 x13: 0000000000000000 x12: ffffffc00a70faa0
| x11: 00000000aaaaaaab x10: 0000000000000054 x9 : ffffffc00839adf8
| x8 : ffffffc009b4cf00 x7 : 0000000000000000 x6 : 0000000000000007
| x5 : 0000000000000000 x4 : 0000000000000000 x3 : ffffffc00a70fb70
| x2 : 0005ff802a0d8d71 x1 : 0000000000000000 x0 : 0000000000000000
| Call trace:
| kcsan_setup_watchpoint+0x26c/0x6bc
| __tsan_read2+0x1f0/0x234
| inflate_fast+0x498/0x750
| zlib_inflate+0x1304/0x2384
| __gunzip+0x3a0/0x45c
| gunzip+0x20/0x30
| unpack_to_rootfs+0x2a8/0x3fc
| do_populate_rootfs+0xe8/0x11c
| async_run_entry_fn+0x58/0x1bc
| process_one_work+0x3ec/0x738
| worker_thread+0x4c4/0x838
| kthread+0x20c/0x258
| ret_from_fork+0x10/0x20
| Code: b8bfc2a8 2a0803f7 14000007 d503249f (78bfc2a8) )
| ---[ end trace 613a943cb0a572b6 ]-----
The reason for this is that on certain arm64 configuration since
e35123d83ee3 ("arm64: lto: Strengthen READ_ONCE() to acquire when
CONFIG_LTO=y"), READ_ONCE() may be promoted to a full atomic acquire
instruction which cannot be used on unaligned addresses.
Fix it by avoiding READ_ONCE() in read_instrumented_memory(), and simply
forcing the compiler to do the required access by casting to the
appropriate volatile type. In terms of generated code this currently
only affects architectures that do not use the default READ_ONCE()
implementation.
The only downside is that we are not guaranteed atomicity of the access
itself, although on most architectures a plain load up to machine word
size should still be atomic (a fact the default READ_ONCE() still relies
on itself).
Reported-by: Haibo Li <haibo.li(a)mediatek.com>
Tested-by: Haibo Li <haibo.li(a)mediatek.com>
Cc: <stable(a)vger.kernel.org> # 5.17+
Signed-off-by: Marco Elver <elver(a)google.com>
---
kernel/kcsan/core.c | 17 +++++++++++++----
1 file changed, 13 insertions(+), 4 deletions(-)
diff --git a/kernel/kcsan/core.c b/kernel/kcsan/core.c
index 54d077e1a2dc..5a60cc52adc0 100644
--- a/kernel/kcsan/core.c
+++ b/kernel/kcsan/core.c
@@ -337,11 +337,20 @@ static void delay_access(int type)
*/
static __always_inline u64 read_instrumented_memory(const volatile void *ptr, size_t size)
{
+ /*
+ * In the below we don't necessarily need the read of the location to
+ * be atomic, and we don't use READ_ONCE(), since all we need for race
+ * detection is to observe 2 different values.
+ *
+ * Furthermore, on certain architectures (such as arm64), READ_ONCE()
+ * may turn into more complex instructions than a plain load that cannot
+ * do unaligned accesses.
+ */
switch (size) {
- case 1: return READ_ONCE(*(const u8 *)ptr);
- case 2: return READ_ONCE(*(const u16 *)ptr);
- case 4: return READ_ONCE(*(const u32 *)ptr);
- case 8: return READ_ONCE(*(const u64 *)ptr);
+ case 1: return *(const volatile u8 *)ptr;
+ case 2: return *(const volatile u16 *)ptr;
+ case 4: return *(const volatile u32 *)ptr;
+ case 8: return *(const volatile u64 *)ptr;
default: return 0; /* Ignore; we do not diff the values. */
}
}
--
2.40.0.rc1.284.g88254d51c5-goog
This reverts commit 487a32ec24be819e747af8c2ab0d5c515508086a.
The should_skip_kasan_poison() function reads the PG_skip_kasan_poison
flag from page->flags. However, this line of code in free_pages_prepare():
page->flags &= ~PAGE_FLAGS_CHECK_AT_PREP;
clears most of page->flags, including PG_skip_kasan_poison, before calling
should_skip_kasan_poison(), which meant that it would never return true
as a result of the page flag being set. Therefore, fix the code to call
should_skip_kasan_poison() before clearing the flags, as we were doing
before the reverted patch.
Signed-off-by: Peter Collingbourne <pcc(a)google.com>
Fixes: 487a32ec24be ("kasan: drop skip_kasan_poison variable in free_pages_prepare")
Cc: <stable(a)vger.kernel.org> # 6.1
Link: https://linux-review.googlesource.com/id/Ic4f13affeebd20548758438bb9ed9ca40…
Reviewed-by: Andrey Konovalov <andreyknvl(a)gmail.com>
---
mm/page_alloc.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index ac1fc986af44..7136c36c5d01 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1398,6 +1398,7 @@ static __always_inline bool free_pages_prepare(struct page *page,
unsigned int order, bool check_free, fpi_t fpi_flags)
{
int bad = 0;
+ bool skip_kasan_poison = should_skip_kasan_poison(page, fpi_flags);
bool init = want_init_on_free();
VM_BUG_ON_PAGE(PageTail(page), page);
@@ -1470,7 +1471,7 @@ static __always_inline bool free_pages_prepare(struct page *page,
* With hardware tag-based KASAN, memory tags must be set before the
* page becomes unavailable via debug_pagealloc or arch_free_page.
*/
- if (!should_skip_kasan_poison(page, fpi_flags)) {
+ if (!skip_kasan_poison) {
kasan_poison_pages(page, order, init);
/* Memory is already initialized if KASAN did it internally. */
--
2.39.2.722.g9855ee24e9-goog