linux-kernel - [PATCH 1/2] mm: krealloc: consider spare memory for __GFP

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Message-ID: <20240730194214.31483-1-dakr@kernel.org>
Date: Tue, 30 Jul 2024 21:42:05 +0200
From: Danilo Krummrich <dakr@...nel.org>
To: akpm@...ux-foundation.org,
	cl@...ux.com,
	penberg@...nel.org,
	rientjes@...gle.com,
	iamjoonsoo.kim@....com,
	vbabka@...e.cz,
	roman.gushchin@...ux.dev,
	42.hyeyoo@...il.com
Cc: linux-kernel@...r.kernel.org,
	linux-mm@...ck.org,
	Danilo Krummrich <dakr@...nel.org>
Subject: [PATCH 1/2] mm: krealloc: consider spare memory for __GFP_ZERO

As long as krealloc() is called with __GFP_ZERO consistently, starting
with the initial memory allocation, __GFP_ZERO should be fully honored.

However, if for an existing allocation krealloc() is called with a
decreased size, it is not ensured that the spare portion the allocation
is zeroed. Thus, if krealloc() is subsequently called with a larger size
again, __GFP_ZERO can't be fully honored, since we don't know the
previous size, but only the bucket size.

Example:

	buf = kzalloc(64, GFP_KERNEL);
	memset(buf, 0xff, 64);

	buf = krealloc(buf, 48, GFP_KERNEL | __GFP_ZERO);

	/* After this call the last 16 bytes are still 0xff. */
	buf = krealloc(buf, 64, GFP_KERNEL | __GFP_ZERO);

Fix this, by explicitly setting spare memory to zero, when shrinking an
allocation with __GFP_ZERO flag set or init_on_alloc enabled.

Signed-off-by: Danilo Krummrich <dakr@...nel.org>
---
 mm/slab_common.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/mm/slab_common.c b/mm/slab_common.c
index 40b582a014b8..cff602cedf8e 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -1273,6 +1273,13 @@ __do_krealloc(const void *p, size_t new_size, gfp_t flags)
 
 	/* If the object still fits, repoison it precisely. */
 	if (ks >= new_size) {
+		/* Zero out spare memory. */
+		if (want_init_on_alloc(flags)) {
+			kasan_disable_current();
+			memset((void *)p + new_size, 0, ks - new_size);
+			kasan_enable_current();
+		}
+
 		p = kasan_krealloc((void *)p, new_size, flags);
 		return (void *)p;
 	}

base-commit: 7c3dd6d99f2df6a9d7944ee8505b195ba51c9b68
-- 
2.45.2