[PATCH 2/2] arm64: io: Extract user memory type in ioremap_prot()

[Will Deacon]

The only caller of ioremap_prot() outside of the generic ioremap()
implementation is generic_access_phys(), which passes a 'pgprot_t' value
determined from the user mapping of the target 'pfn' being accessed by
the kernel. On arm64, the 'pgprot_t' contains all of the non-address
bits from the pte, including the permission controls, and so we end up
returning a new user mapping from ioremap_prot() which faults when
accessed from the kernel on systems with PAN:

  | Unable to handle kernel read from unreadable memory at virtual address ffff80008ea89000
  | ...
  | Call trace:
  |   __memcpy_fromio+0x80/0xf8
  |   generic_access_phys+0x20c/0x2b8
  |   __access_remote_vm+0x46c/0x5b8
  |   access_remote_vm+0x18/0x30
  |   environ_read+0x238/0x3e8
  |   vfs_read+0xe4/0x2b0
  |   ksys_read+0xcc/0x178
  |   __arm64_sys_read+0x4c/0x68

Extract only the memory type from the user 'pgprot_t' in ioremap_prot()
and assert that we're being passed a user mapping, to protect us against
any changes in future that may require additional handling. To avoid
falsely flagging users of ioremap(), provide our own ioremap() macro
which simply wraps __ioremap_prot().

Cc: Zeng Heng <zengheng4 at huawei.com>
Cc: Jinjiang Tu <tujinjiang at huawei.com>
Cc: Catalin Marinas <catalin.marinas at arm.com>
Fixes: 893dea9ccd08 ("arm64: Add HAVE_IOREMAP_PROT support")
Reported-by: Jinjiang Tu <tujinjiang at huawei.com>
Signed-off-by: Will Deacon <will at kernel.org>
---
 arch/arm64/include/asm/io.h | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/include/asm/io.h b/arch/arm64/include/asm/io.h
index cd2fddfe814a..8cbd1e96fd50 100644
--- a/arch/arm64/include/asm/io.h
+++ b/arch/arm64/include/asm/io.h
@@ -266,10 +266,23 @@ typedef int (*ioremap_prot_hook_t)(phys_addr_t phys_addr, size_t size,
 int arm64_ioremap_prot_hook_register(const ioremap_prot_hook_t hook);
 void __iomem *__ioremap_prot(phys_addr_t phys, size_t size, pgprot_t prot);
 
-#define ioremap_prot __ioremap_prot
+static inline void __iomem *ioremap_prot(phys_addr_t phys, size_t size,
+					 pgprot_t user_prot)
+{
+	pgprot_t prot;
+	ptdesc_t user_prot_val = pgprot_val(user_prot);
 
-#define _PAGE_IOREMAP PROT_DEVICE_nGnRE
+	if (WARN_ON_ONCE(!(user_prot_val & PTE_USER)))
+		return NULL;
 
+	prot = __pgprot_modify(PAGE_KERNEL, PTE_ATTRINDX_MASK,
+			       user_prot_val & PTE_ATTRINDX_MASK);
+	return __ioremap_prot(phys, size, prot);
+}
+#define ioremap_prot ioremap_prot
+
+#define ioremap(addr, size)	\
+	__ioremap_prot((addr), (size), __pgprot(PROT_DEVICE_nGnRE))
 #define ioremap_wc(addr, size)	\
 	__ioremap_prot((addr), (size), __pgprot(PROT_NORMAL_NC))
 #define ioremap_np(addr, size)	\
-- 
2.53.0.371.g1d285c8824-goog