[PATCH 7.0.y] iommu/amd: Remove latent out-of-bounds access in IOMMU debugfs

[Eder Zulian <ezulian@redhat.com>]

In iommu_mmio_write() and iommu_capability_write(),
iommu->dbg_mmio_offset and iommu->dbg_cap_offset are int. However, they
are populated using kstrtou32_from_user(). If a user provides a
sufficiently large value, it can become a negative integer.

Prior to this patch, the AMD IOMMU debugfs implementation was already
protected by different mechanisms.

1. #define OFS_IN_SZ 8 ensures the user string <= 8 bytes, so
   e.g. 0xffffffff isn't a valid input.

  if (cnt > OFS_IN_SZ)
     return -EINVAL;

2. Implicit type promotion in iommu_mmio_write(), iommu->dbg_mmio_offset
   is int and iommu->mmio_phys_end is u64

  if (iommu->dbg_mmio_offset > iommu->mmio_phys_end - sizeof(u64))
      return -EINVAL;

3. The show handlers would currently catch the negative number and
   refuse to perform the read.

Replace kstrtou32_from_user() with kstrtos32_from_user() to parse the
input, and check for negative values to explicitly prevent out-of-bounds
memory accesses directly in iommu_mmio_write() and
iommu_capability_write().

Signed-off-by: Eder Zulian <ezulian@redhat.com>
Fixes: 7a4ee419e8c1 ("iommu/amd: Add debugfs support to dump IOMMU MMIO registers")
Cc: stable@vger.kernel.org
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
(cherry picked from commit 8dfd3d8d74435344ee8dc9237596959c8b2a6cbe)
---
 drivers/iommu/amd/debugfs.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/drivers/iommu/amd/debugfs.c b/drivers/iommu/amd/debugfs.c
index 20b04996441d..bb0c8552bc0f 100644
--- a/drivers/iommu/amd/debugfs.c
+++ b/drivers/iommu/amd/debugfs.c
@@ -33,11 +33,12 @@ static ssize_t iommu_mmio_write(struct file *filp, const char __user *ubuf,
 	if (cnt > OFS_IN_SZ)
 		return -EINVAL;
 
-	ret = kstrtou32_from_user(ubuf, cnt, 0, &iommu->dbg_mmio_offset);
+	ret = kstrtos32_from_user(ubuf, cnt, 0, &iommu->dbg_mmio_offset);
 	if (ret)
 		return ret;
 
-	if (iommu->dbg_mmio_offset > iommu->mmio_phys_end - sizeof(u64)) {
+	if (iommu->dbg_mmio_offset < 0 || iommu->dbg_mmio_offset >
+			iommu->mmio_phys_end - sizeof(u64)) {
 		iommu->dbg_mmio_offset = -1;
 		return  -EINVAL;
 	}
@@ -74,12 +75,12 @@ static ssize_t iommu_capability_write(struct file *filp, const char __user *ubuf
 	if (cnt > OFS_IN_SZ)
 		return -EINVAL;
 
-	ret = kstrtou32_from_user(ubuf, cnt, 0, &iommu->dbg_cap_offset);
+	ret = kstrtos32_from_user(ubuf, cnt, 0, &iommu->dbg_cap_offset);
 	if (ret)
 		return ret;
 
 	/* Capability register at offset 0x14 is the last IOMMU capability register. */
-	if (iommu->dbg_cap_offset > 0x14) {
+	if (iommu->dbg_cap_offset < 0 || iommu->dbg_cap_offset > 0x14) {
 		iommu->dbg_cap_offset = -1;
 		return -EINVAL;
 	}
-- 
2.54.0