iommu/amd: Remove latent out-of-bounds access in IOMMU debugfs

In iommu_mmio_write() and iommu_capability_write(), the variables
dbg_mmio_offset and dbg_cap_offset are declared as 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(), dbg_mmio_offset is int
   and iommu->mmio_phys_end is u64

  if (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>

diff --git a/drivers/iommu/amd/debugfs.c b/drivers/iommu/amd/debugfs.c
index 4e66473..4c53b63 100644 (file)
--- a/drivers/iommu/amd/debugfs.c
+++ b/drivers/iommu/amd/debugfs.c
@@ -31,11 +31,12 @@ 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, &dbg_mmio_offset);
+       ret = kstrtos32_from_user(ubuf, cnt, 0, &dbg_mmio_offset);
        if (ret)
                return ret;
 
-       if (dbg_mmio_offset > iommu->mmio_phys_end - sizeof(u64))
+       if (dbg_mmio_offset < 0 || dbg_mmio_offset >
+                       iommu->mmio_phys_end - sizeof(u64))
                return -EINVAL;
 
        iommu->dbg_mmio_offset = dbg_mmio_offset;
@@ -71,12 +72,12 @@ 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, &dbg_cap_offset);
+       ret = kstrtos32_from_user(ubuf, cnt, 0, &dbg_cap_offset);
        if (ret)
                return ret;
 
        /* Capability register at offset 0x14 is the last IOMMU capability register. */
-       if (dbg_cap_offset > 0x14)
+       if (dbg_cap_offset < 0 || dbg_cap_offset > 0x14)
                return -EINVAL;
 
        iommu->dbg_cap_offset = dbg_cap_offset;