+ mm-krealloc-consider-spare-memory-for-__gfp_zero.patch added to mm-unstable branch

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: mm: krealloc: consider spare memory for __GFP_ZERO
has been added to the -mm mm-unstable branch.  Its filename is
     mm-krealloc-consider-spare-memory-for-__gfp_zero.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-krealloc-consider-spare-memory-for-__gfp_zero.patch

This patch will later appear in the mm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Danilo Krummrich <dakr@kernel.org>
Subject: mm: krealloc: consider spare memory for __GFP_ZERO
Date: Tue, 13 Aug 2024 00:34:34 +0200

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.

Link: https://lkml.kernel.org/r/20240812223707.32049-1-dakr@kernel.org
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: David Rientjes <rientjes@google.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/slab_common.c |    7 +++++++
 1 file changed, 7 insertions(+)

--- a/mm/slab_common.c~mm-krealloc-consider-spare-memory-for-__gfp_zero
+++ a/mm/slab_common.c
@@ -1273,6 +1273,13 @@ __do_krealloc(const void *p, size_t new_
 
 	/* 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;
 	}
_

Patches currently in -mm which might be from dakr@kernel.org are

mm-vmalloc-implement-vrealloc.patch
mm-vmalloc-implement-vrealloc-fix.patch
mm-vmalloc-implement-vrealloc-fix-2.patch
mm-vmalloc-implement-vrealloc-fix-3.patch
mm-vmalloc-implement-vrealloc-fix-4.patch
mm-kvmalloc-align-kvrealloc-with-krealloc.patch
mm-kvmalloc-align-kvrealloc-with-krealloc-fix.patch
mm-kvmalloc-align-kvrealloc-with-krealloc-fix-2.patch
mm-kvmalloc-align-kvrealloc-with-krealloc-fix-3.patch
mm-krealloc-consider-spare-memory-for-__gfp_zero.patch
mm-krealloc-clarify-valid-usage-of-__gfp_zero.patch