[PATCH v6 3/6] mm/vmalloc: zero newly exposed memory on vrealloc() grow

[Shivam Kalra via B4 Relay <devnull+shivamkalra98.zohomail.in@kernel.org>]

From: Shivam Kalra <shivamkalra98@zohomail.in>

When growing an existing vmalloc allocation in-place, zero the newly
exposed memory region [old_size, size) if the caller requested it via
__GFP_ZERO (checked via want_init_on_alloc(flags)).

Previously, the code assumed that the unused capacity in the vm_struct
was already zeroed either at initial allocation time or during a prior
shrink. However, if an intermediate shrink operation occurred without
__GFP_ZERO and without init_on_free enabled, the "freed" portion of the
allocation would retain its old data.

If a subsequent grow-in-place operation then explicitly requests
__GFP_ZERO, failing to zero the memory here would violate the allocation
flags and leak the previously discarded, potentially sensitive data.

Signed-off-by: Shivam Kalra <shivamkalra98@zohomail.in>
---
 mm/vmalloc.c | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 7bc7a6892c1a..64f5d1088281 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -4362,13 +4362,16 @@ void *vrealloc_node_align_noprof(const void *p, size_t size, unsigned long align
 	 * We already have the bytes available in the allocation; use them.
 	 */
 	if (size <= (size_t)vm->nr_pages << PAGE_SHIFT) {
-		/*
-		 * No need to zero memory here, as unused memory will have
-		 * already been zeroed at initial allocation time or during
-		 * realloc shrink time.
-		 */
 		vm->requested_size = size;
 		kasan_vrealloc(p, old_size, size);
+
+		/*
+		 * Zero the newly exposed bytes if requested.
+		 * The region [old_size, size) may contain stale data from
+		 * a previous shrink that did not use __GFP_ZERO.
+		 */
+		if (want_init_on_alloc(flags))
+			memset((void *)p + old_size, 0, size - old_size);
 		return (void *)p;
 	}
 

-- 
2.43.0