[PATCH] iommu/amd: Fix potential out-of-bounds read in iommu_mmio_show

[Songtang Liu]

In iommu_mmio_write(), it validates the user-provided offset with the
check: `iommu->dbg_mmio_offset > iommu->mmio_phys_end - 4`.
This assumes a 4-byte access. However, the corresponding
show handler, iommu_mmio_show(), uses readq() to perform an 8-byte
(64-bit) read.

If a user provides an offset equal to `mmio_phys_end - 4`, the check
passes, and will lead to a 4-byte out-of-bounds read.

Fix this by adjusting the boundary check to use sizeof(u64), which
corresponds to the size of the readq() operation.

Fixes: 7a4ee419e8c1 ("iommu/amd: Add debugfs support to dump IOMMU
MMIO registers")
Signed-off-by: Songtang Liu <liusongtang@xxxxxxxxxxxxx>
---
 drivers/iommu/amd/debugfs.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/iommu/amd/debugfs.c b/drivers/iommu/amd/debugfs.c
index 8b4d7db7869a4..5d85c47478a57 100644
--- a/drivers/iommu/amd/debugfs.c
+++ b/drivers/iommu/amd/debugfs.c
@@ -37,7 +37,7 @@ static ssize_t iommu_mmio_write(struct file *filp,
const char __user *ubuf,
 	if (ret)
 		return ret;

-	if (iommu->dbg_mmio_offset > iommu->mmio_phys_end - 4) {
+	if (iommu->dbg_mmio_offset > iommu->mmio_phys_end - sizeof(u64)) {
 		iommu->dbg_mmio_offset = -1;
 		return  -EINVAL;
 	}
-- 
2.39.5