x86/sev-es: Use __put_user()/__get_user() for data accesses

The put_user() and get_user() functions do checks on the address which is
passed to them. They check whether the address is actually a user-space
address and whether its fine to access it. They also call might_fault()
to indicate that they could fault and possibly sleep.

All of these checks are neither wanted nor needed in the #VC exception
handler, which can be invoked from almost any context and also for MMIO
instructions from kernel space on kernel memory. All the #VC handler
wants to know is whether a fault happened when the access was tried.

This is provided by __put_user()/__get_user(), which just do the access
no matter what. Also add comments explaining why __get_user() and
__put_user() are the best choice here and why it is safe to use them
in this context. Also explain why copy_to/from_user can't be used.

In addition, also revert commit

  7024f60d6552 ("x86/sev-es: Handle string port IO to kernel memory properly")

because using __get_user()/__put_user() fixes the same problem while
the above commit introduced several problems:

  1) It uses access_ok() which is only allowed in task context.

  2) It uses memcpy() which has no fault handling at all and is
     thus unsafe to use here.

  [ bp: Fix up commit ID of the reverted commit above. ]

Fixes: f980f9c31a92 ("x86/sev-es: Compile early handler code into kernel image")
Signed-off-by: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: stable@vger.kernel.org # v5.10+
Link: https://lkml.kernel.org/r/20210519135251.30093-4-joro@8bytes.org

diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
index 1f428f401bedab1d7a9b5be233652631595a1515..651b81cd648e5dc183853d90e2d5f1c33d39ed50 100644 (file)
--- a/arch/x86/kernel/sev.c
+++ b/arch/x86/kernel/sev.c
@@ -315,31 +315,44 @@ static enum es_result vc_write_mem(struct es_em_ctxt *ctxt,
        u16 d2;
        u8  d1;
 
-       /* If instruction ran in kernel mode and the I/O buffer is in kernel space */
-       if (!user_mode(ctxt->regs) && !access_ok(target, size)) {
-               memcpy(dst, buf, size);
-               return ES_OK;
-       }
-
+       /*
+        * This function uses __put_user() independent of whether kernel or user
+        * memory is accessed. This works fine because __put_user() does no
+        * sanity checks of the pointer being accessed. All that it does is
+        * to report when the access failed.
+        *
+        * Also, this function runs in atomic context, so __put_user() is not
+        * allowed to sleep. The page-fault handler detects that it is running
+        * in atomic context and will not try to take mmap_sem and handle the
+        * fault, so additional pagefault_enable()/disable() calls are not
+        * needed.
+        *
+        * The access can't be done via copy_to_user() here because
+        * vc_write_mem() must not use string instructions to access unsafe
+        * memory. The reason is that MOVS is emulated by the #VC handler by
+        * splitting the move up into a read and a write and taking a nested #VC
+        * exception on whatever of them is the MMIO access. Using string
+        * instructions here would cause infinite nesting.
+        */
        switch (size) {
        case 1:
                memcpy(&d1, buf, 1);
-               if (put_user(d1, target))
+               if (__put_user(d1, target))
                        goto fault;
                break;
        case 2:
                memcpy(&d2, buf, 2);
-               if (put_user(d2, target))
+               if (__put_user(d2, target))
                        goto fault;
                break;
        case 4:
                memcpy(&d4, buf, 4);
-               if (put_user(d4, target))
+               if (__put_user(d4, target))
                        goto fault;
                break;
        case 8:
                memcpy(&d8, buf, 8);
-               if (put_user(d8, target))
+               if (__put_user(d8, target))
                        goto fault;
                break;
        default:
@@ -370,30 +383,43 @@ static enum es_result vc_read_mem(struct es_em_ctxt *ctxt,
        u16 d2;
        u8  d1;
 
-       /* If instruction ran in kernel mode and the I/O buffer is in kernel space */
-       if (!user_mode(ctxt->regs) && !access_ok(s, size)) {
-               memcpy(buf, src, size);
-               return ES_OK;
-       }
-
+       /*
+        * This function uses __get_user() independent of whether kernel or user
+        * memory is accessed. This works fine because __get_user() does no
+        * sanity checks of the pointer being accessed. All that it does is
+        * to report when the access failed.
+        *
+        * Also, this function runs in atomic context, so __get_user() is not
+        * allowed to sleep. The page-fault handler detects that it is running
+        * in atomic context and will not try to take mmap_sem and handle the
+        * fault, so additional pagefault_enable()/disable() calls are not
+        * needed.
+        *
+        * The access can't be done via copy_from_user() here because
+        * vc_read_mem() must not use string instructions to access unsafe
+        * memory. The reason is that MOVS is emulated by the #VC handler by
+        * splitting the move up into a read and a write and taking a nested #VC
+        * exception on whatever of them is the MMIO access. Using string
+        * instructions here would cause infinite nesting.
+        */
        switch (size) {
        case 1:
-               if (get_user(d1, s))
+               if (__get_user(d1, s))
                        goto fault;
                memcpy(buf, &d1, 1);
                break;
        case 2:
-               if (get_user(d2, s))
+               if (__get_user(d2, s))
                        goto fault;
                memcpy(buf, &d2, 2);
                break;
        case 4:
-               if (get_user(d4, s))
+               if (__get_user(d4, s))
                        goto fault;
                memcpy(buf, &d4, 4);
                break;
        case 8:
-               if (get_user(d8, s))
+               if (__get_user(d8, s))
                        goto fault;
                memcpy(buf, &d8, 8);
                break;