linux-kernel - [PATCH 1/1] drivers/char/mem.c: Disable encryption bit in page tables for, MMIO access

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Date:   Thu, 4 Oct 2018 13:18:58 -0400
From:   James Puthukattukaran <james.puthukattukaran@...cle.com>
To:     "Lendacky, Thomas" <thomas.lendacky@....com>,
        "Singh, Brijesh" <brijesh.singh@....com>
Cc:     gregkh@...uxfoundation.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: [PATCH 1/1] drivers/char/mem.c: Disable encryption bit in page tables
 for, MMIO access

Attempting to mmap to a memory mapped IO space returns -1s because the
memory encryption bit is set for these pages. According to the AMD spec,
this bit should not be set for non-DRAM space. The patch checks if this
is an memory IO region being accessed and decrypts accordingly.

Signed-off-by: James Puthukattukaran<james.puthukattukaran@...cle.com>
---
 drivers/char/mem.c | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/drivers/char/mem.c b/drivers/char/mem.c
index ffeb60d..beaa374 100644
--- a/drivers/char/mem.c
+++ b/drivers/char/mem.c
@@ -355,10 +355,30 @@ static inline int private_mapping_ok(struct vm_area_struct *vma)
 #endif
 };
 
+static int check_flags(struct resource *res, void *arg)
+{
+	int *mmio = arg;
+	*mmio = (!((res->flags & IORESOURCE_SYSTEM_RAM) ==
+		IORESOURCE_SYSTEM_RAM) && (res->desc == IORES_DESC_NONE));
+	return *mmio;
+}
+
+static void check_iomem_region(phys_addr_t addr, size_t size,
+				int *mmio)
+{
+	u64 start, end;
+
+	start = (u64)addr;
+	end = start + size - 1;
+	*mmio = 0;
+	walk_mem_res(start, end, mmio, check_flags);
+}
+
 static int mmap_mem(struct file *file, struct vm_area_struct *vma)
 {
 	size_t size = vma->vm_end - vma->vm_start;
 	phys_addr_t offset = (phys_addr_t)vma->vm_pgoff << PAGE_SHIFT;
+	int mmio;
 
 	/* Does it even fit in phys_addr_t? */
 	if (offset >> PAGE_SHIFT != vma->vm_pgoff)
@@ -387,6 +407,13 @@ static int mmap_mem(struct file *file, struct vm_area_struct *vma)
 
 	vma->vm_ops = &mmap_mem_ops;
 
+	if (mem_encrypt_active()) {
+		check_iomem_region(vma->vm_pgoff, size, &mmio);
+		if (mmio)
+			vma->vm_page_prot  =
+					pgprot_decrypted(vma->vm_page_prot);
+	}
+
 	/* Remap-pfn-range will mark the range VM_IO */
 	if (remap_pfn_range(vma,
 			    vma->vm_start,