linux-kernel - [PATCH 5.13 196/223] kfence: move the size check to the beginning of __kfence

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Date:   Mon, 26 Jul 2021 17:39:48 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Alexander Potapenko <glider@...gle.com>,
        Marco Elver <elver@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [PATCH 5.13 196/223] kfence: move the size check to the beginning of __kfence_alloc()

From: Alexander Potapenko <glider@...gle.com>

commit 235a85cb32bb123854ad31de46fdbf04c1d57cda upstream.

Check the allocation size before toggling kfence_allocation_gate.

This way allocations that can't be served by KFENCE will not result in
waiting for another CONFIG_KFENCE_SAMPLE_INTERVAL without allocating
anything.

Link: https://lkml.kernel.org/r/20210714092222.1890268-1-glider@google.com
Signed-off-by: Alexander Potapenko <glider@...gle.com>
Suggested-by: Marco Elver <elver@...gle.com>
Reviewed-by: Marco Elver <elver@...gle.com>
Cc: Dmitry Vyukov <dvyukov@...gle.com>
Cc: Marco Elver <elver@...gle.com>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: <stable@...r.kernel.org>	[5.12+]
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 mm/kfence/core.c |   10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

--- a/mm/kfence/core.c
+++ b/mm/kfence/core.c
@@ -734,6 +734,13 @@ void kfence_shutdown_cache(struct kmem_c
 void *__kfence_alloc(struct kmem_cache *s, size_t size, gfp_t flags)
 {
 	/*
+	 * Perform size check before switching kfence_allocation_gate, so that
+	 * we don't disable KFENCE without making an allocation.
+	 */
+	if (size > PAGE_SIZE)
+		return NULL;
+
+	/*
 	 * allocation_gate only needs to become non-zero, so it doesn't make
 	 * sense to continue writing to it and pay the associated contention
 	 * cost, in case we have a large number of concurrent allocations.
@@ -757,9 +764,6 @@ void *__kfence_alloc(struct kmem_cache *
 	if (!READ_ONCE(kfence_enabled))
 		return NULL;
 
-	if (size > PAGE_SIZE)
-		return NULL;
-
 	return kfence_guarded_alloc(s, size, flags);
 }