x86/mm/64: Do not dereference non-present PGD entries

The code for preallocate_vmalloc_pages() was written under the
assumption that the p4d_offset() and pud_offset() functions will perform
present checks before dereferencing the parent entries.

This assumption is wrong an leads to a bug in the code which causes the
physical address found in the PGD be used as a page-table page, even if
the PGD is not present.

So the code flow currently is:

	pgd = pgd_offset_k(addr);
	p4d = p4d_offset(pgd, addr);
	if (p4d_none(*p4d))
		p4d = p4d_alloc(&init_mm, pgd, addr);

This lacks a check for pgd_none() at least, the correct flow would be:

	pgd = pgd_offset_k(addr);
	if (pgd_none(*pgd))
		p4d = p4d_alloc(&init_mm, pgd, addr);
	else
		p4d = p4d_offset(pgd, addr);

But this is the same flow that the p4d_alloc() and the pud_alloc()
functions use internally, so there is no need to duplicate them.

Remove the p?d_none() checks from the function and just call into
p4d_alloc() and pud_alloc() to correctly pre-allocate the PGD entries.

Reported-and-tested-by: Jason A. Donenfeld <Jason@zx2c4.com>
Reviewed-by: Mike Rapoport <rppt@linux.ibm.com>
Fixes: 6eb82f994026 ("x86/mm: Pre-allocate P4D/PUD pages for vmalloc area")
Signed-off-by: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
index 3f4e29a..449e071 100644 (file)
--- a/arch/x86/mm/init_64.c
+++ b/arch/x86/mm/init_64.c
@@ -1253,28 +1253,23 @@ static void __init preallocate_vmalloc_pages(void)
                p4d_t *p4d;
                pud_t *pud;
 
-               p4d = p4d_offset(pgd, addr);
-               if (p4d_none(*p4d)) {
-                       /* Can only happen with 5-level paging */
-                       p4d = p4d_alloc(&init_mm, pgd, addr);
-                       if (!p4d) {
-                               lvl = "p4d";
-                               goto failed;
-                       }
-               }
+               lvl = "p4d";
+               p4d = p4d_alloc(&init_mm, pgd, addr);
+               if (!p4d)
+                       goto failed;
 
+               /*
+                * With 5-level paging the P4D level is not folded. So the PGDs
+                * are now populated and there is no need to walk down to the
+                * PUD level.
+                */
                if (pgtable_l5_enabled())
                        continue;
 
-               pud = pud_offset(p4d, addr);
-               if (pud_none(*pud)) {
-                       /* Ends up here only with 4-level paging */
-                       pud = pud_alloc(&init_mm, p4d, addr);
-                       if (!pud) {
-                               lvl = "pud";
-                               goto failed;
-                       }
-               }
+               lvl = "pud";
+               pud = pud_alloc(&init_mm, p4d, addr);
+               if (!pud)
+                       goto failed;
        }
 
        return;