[PATCH v1 2/4] crypto/ccp: Fix snp_filter_reserved_mem_regions() off-by-one

[Tycho Andersen <tycho@kernel.org>]

From: "Tycho Andersen (AMD)" <tycho@kernel.org>

Sashiko notes:

> regarding the bounds check in snp_filter_reserved_mem_regions()
> called via walk_iomem_res_desc(): does the check
> if ((range_list->num_elements * 16 + 8) > PAGE_SIZE)
> allow an off-by-one heap buffer overflow?
>
> If range_list->num_elements is 255, 255 * 16 + 8 = 4088, which is <= 4096.
> Writing range->base (8 bytes) fills 4088-4095, but writing range->page_count
> (4 bytes) would write to 4096-4099, overflowing the kzalloc-allocated
> PAGE_SIZE buffer.

Fix this by accounting for the entry about to be written to, in addition to
the entries that are already allocated.

Fixes: 1ca5614b84ee ("crypto: ccp: Add support to initialize the AMD-SP for SEV-SNP")
Reported-by: Sashiko
Assisted-by: Gemini:gemini-3.1-pro-preview
Link: https://sashiko.dev/#/patchset/20260324161301.1353976-1-tycho%40kernel.org
Signed-off-by: Tycho Andersen (AMD) <tycho@kernel.org>
---
 drivers/crypto/ccp/sev-dev.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/drivers/crypto/ccp/sev-dev.c b/drivers/crypto/ccp/sev-dev.c
index 939fa8aa155c..e87efcff8df2 100644
--- a/drivers/crypto/ccp/sev-dev.c
+++ b/drivers/crypto/ccp/sev-dev.c
@@ -1328,10 +1328,11 @@ static int snp_filter_reserved_mem_regions(struct resource *rs, void *arg)
 	size_t size;
 
 	/*
-	 * Ensure the list of HV_FIXED pages that will be passed to firmware
-	 * do not exceed the page-sized argument buffer.
+	 * Ensure the list of HV_FIXED pages passed to the firmware including
+	 * the one about to be written to do not exceed the page-sized argument
+	 * buffer.
 	 */
-	if ((range_list->num_elements * sizeof(struct sev_data_range) +
+	if (((range_list->num_elements + 1) * sizeof(struct sev_data_range) +
 	     sizeof(struct sev_data_range_list)) > PAGE_SIZE)
 		return -E2BIG;
 
-- 
2.53.0