[PATCH v3] accel/ethosu: fix arithmetic issues in dma_length()

[Muhammad Bilal <meatuni001@gmail.com>]

dma_length() derives DMA region usage from command stream values and
updates region_size[]:

    len = ((len + stride[0]) * size0 + stride[1]) * size1
    region_size[region] = max(..., len + dma->offset)

Several arithmetic issues can corrupt the derived region size:

- signed stride values may underflow when added to len
- intermediate multiplications may overflow
- len + dma->offset may overflow during region_size updates
- dma_length() error returns were not validated by the caller

region_size[] is later used by ethosu_job.c to validate command stream
accesses against GEM buffer sizes. Arithmetic wraparound can therefore
under-report region usage and bypass the bounds validation.

Fix by validating signed additions, using overflow helpers for
multiplications and offset updates, and propagating dma_length()
failures to the caller.

Fixes: 5a5e9c0228e6 ("accel: Add Arm Ethos-U NPU driver")
Cc: stable@vger.kernel.org
Signed-off-by: Muhammad Bilal <meatuni001@gmail.com>
---

v3:
- add check_add_overflow() for len + dma->offset
- validate dma_length() return value in caller
- rework commit message to avoid unproven claims

v2:
- add negative stride underflow checks before each addition
- replace unchecked multiplications with check_mul_overflow()
 drivers/accel/ethosu/ethosu_gem.c | 23 ++++++++++++++++++-----
 1 file changed, 18 insertions(+), 5 deletions(-)

diff --git a/drivers/accel/ethosu/ethosu_gem.c b/drivers/accel/ethosu/ethosu_gem.c
index 5a02285a4986..8e95539da98f 100644
--- a/drivers/accel/ethosu/ethosu_gem.c
+++ b/drivers/accel/ethosu/ethosu_gem.c
@@ -2,6 +2,7 @@
 /* Copyright 2025 Arm, Ltd. */
 
 #include <linux/err.h>
+#include <linux/overflow.h>
 #include <linux/slab.h>
 
 #include <drm/ethosu_accel.h>
@@ -164,16 +165,26 @@ static u64 dma_length(struct ethosu_validated_cmdstream_info *info,
 	u64 len = dma->len;
 
 	if (mode >= 1) {
+		if (dma->stride[0] < 0 && (u64)(-dma->stride[0]) > len)
+			return U64_MAX;
 		len += dma->stride[0];
-		len *= dma_st->size0;
+		if (check_mul_overflow(len, (u64)dma_st->size0, &len))
+			return U64_MAX;
 	}
 	if (mode == 2) {
+		if (dma->stride[1] < 0 && (u64)(-dma->stride[1]) > len)
+			return U64_MAX;
 		len += dma->stride[1];
-		len *= dma_st->size1;
+		if (check_mul_overflow(len, (u64)dma_st->size1, &len))
+			return U64_MAX;
+	}
+	if (dma->region >= 0) {
+		u64 end;
+
+		if (check_add_overflow(len, dma->offset, &end))
+			return U64_MAX;
+		info->region_size[dma->region] = max(info->region_size[dma->region], end);
 	}
-	if (dma->region >= 0)
-		info->region_size[dma->region] = max(info->region_size[dma->region],
-						     len + dma->offset);
 
 	return len;
 }
@@ -397,6 +408,8 @@ static int ethosu_gem_cmdstream_copy_and_validate(struct drm_device *ddev,
 		case NPU_OP_DMA_START:
 			srclen = dma_length(info, &st.dma, &st.dma.src);
 			dstlen = dma_length(info, &st.dma, &st.dma.dst);
+			if (srclen == U64_MAX || dstlen == U64_MAX)
+				return -EINVAL;
 
 			if (st.dma.dst.region >= 0)
 				info->output_region[st.dma.dst.region] = true;
-- 
2.53.0