Without this patch any kernel code that reads kernel memory in non
present kernel pte/pmds (as set by pageattr.c) will crash.
With this kernel code:
static struct page *crash_page;
static unsigned long *crash_address;
[..]
crash_page = alloc_pages(GFP_KERNEL, 9);
crash_address = page_address(crash_page);
if (set_memory_np((unsigned long)crash_address, 1))
printk("set_memory_np failure\n");
[..]
The kernel will crash if inside the "crash tool" one would try to read
the memory at the not present address.
crash> p crash_address
crash_address = $8 = (long unsigned int *) 0xffff88023c000000
crash> rd 0xffff88023c000000
[ *lockup* ]
The lockup happens because _PAGE_GLOBAL and _PAGE_PROTNONE shares the
same bit, and pageattr leaves _PAGE_GLOBAL set on a kernel pte which
is then mistaken as _PAGE_PROTNONE (so pte_present returns true by
mistake and the kernel fault then gets confused and loops).
With THP the same can happen after we taught pmd_present to check
_PAGE_PROTNONE and _PAGE_PSE in commit
027ef6c87853b0a9df53175063028edb4950d476. THP has the same problem
with _PAGE_GLOBAL as the 4k pages, but it also has a problem with
_PAGE_PSE, which must be cleared too.
After the patch is applied copy_user correctly returns -EFAULT and
doesn't lockup anymore.
crash> p crash_address
crash_address = $9 = (long unsigned int *) 0xffff88023c000000
crash> rd 0xffff88023c000000
rd: read error: kernel virtual address: ffff88023c000000 type: "64-bit KVADDR"
Signed-off-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
---
arch/x86/mm/pageattr.c | 50 +++++++++++++++++++++++++++++++++++++++++++++--
1 files changed, 47 insertions(+), 3 deletions(-)
diff --git a/arch/x86/mm/pageattr.c b/arch/x86/mm/pageattr.c
index a718e0d..2713be4 100644
--- a/arch/x86/mm/pageattr.c
+++ b/arch/x86/mm/pageattr.c
@@ -445,6 +445,19 @@ try_preserve_large_page(pte_t *kpte, unsigned long address,
pgprot_val(req_prot) |= pgprot_val(cpa->mask_set);
/*
+ * Set the PSE and GLOBAL flags only if the PRESENT flag is
+ * set otherwise pmd_present/pmd_huge will return true even on
+ * a non present pmd. The canon_pgprot will clear _PAGE_GLOBAL
+ * for the ancient hardware that doesn't support it.
+ */
+ if (pgprot_val(new_prot) & _PAGE_PRESENT)
+ pgprot_val(new_prot) |= _PAGE_PSE | _PAGE_GLOBAL;
+ else
+ pgprot_val(new_prot) &= ~(_PAGE_PSE | _PAGE_GLOBAL);
+
+ new_prot = canon_pgprot(new_prot);
+
+ /*
* old_pte points to the large page base address. So we need
* to add the offset of the virtual address:
*/
@@ -489,7 +502,7 @@ try_preserve_large_page(pte_t *kpte, unsigned long address,
* The address is aligned and the number of pages
* covers the full page.
*/
- new_pte = pfn_pte(pte_pfn(old_pte), canon_pgprot(new_prot));
+ new_pte = pfn_pte(pte_pfn(old_pte), new_prot);
__set_pmd_pte(kpte, address, new_pte);
cpa->flags |= CPA_FLUSHTLB;
do_split = 0;
@@ -540,16 +553,35 @@ static int split_large_page(pte_t *kpte, unsigned long address)
#ifdef CONFIG_X86_64
if (level == PG_LEVEL_1G) {
pfninc = PMD_PAGE_SIZE >> PAGE_SHIFT;
- pgprot_val(ref_prot) |= _PAGE_PSE;
+ /*
+ * Set the PSE flags only if the PRESENT flag is set
+ * otherwise pmd_present/pmd_huge will return true
+ * even on a non present pmd.
+ */
+ if (pgprot_val(ref_prot) & _PAGE_PRESENT)
+ pgprot_val(ref_prot) |= _PAGE_PSE;
+ else
+ pgprot_val(ref_prot) &= ~_PAGE_PSE;
}
#endif
/*
+ * Set the GLOBAL flags only if the PRESENT flag is set
+ * otherwise pmd/pte_present will return true even on a non
+ * present pmd/pte. The canon_pgprot will clear _PAGE_GLOBAL
+ * for the ancient hardware that doesn't support it.
+ */
+ if (pgprot_val(ref_prot) & _PAGE_PRESENT)
+ pgprot_val(ref_prot) |= _PAGE_GLOBAL;
+ else
+ pgprot_val(ref_prot) &= ~_PAGE_GLOBAL;
+
+ /*
* Get the target pfn from the original entry:
*/
pfn = pte_pfn(*kpte);
for (i = 0; i < PTRS_PER_PTE; i++, pfn += pfninc)
- set_pte(&pbase[i], pfn_pte(pfn, ref_prot));
+ set_pte(&pbase[i], pfn_pte(pfn, canon_pgprot(ref_prot)));
if (address >= (unsigned long)__va(0) &&
address < (unsigned long)__va(max_low_pfn_mapped << PAGE_SHIFT))
@@ -660,6 +692,18 @@ repeat:
new_prot = static_protections(new_prot, address, pfn);
/*
+ * Set the GLOBAL flags only if the PRESENT flag is
+ * set otherwise pte_present will return true even on
+ * a non present pte. The canon_pgprot will clear
+ * _PAGE_GLOBAL for the ancient hardware that doesn't
+ * support it.
+ */
+ if (pgprot_val(new_prot) & _PAGE_PRESENT)
+ pgprot_val(new_prot) |= _PAGE_GLOBAL;
+ else
+ pgprot_val(new_prot) &= ~_PAGE_GLOBAL;
+
+ /*
* We need to keep the pfn from the existing PTE,
* after all we're only going to change it's attributes
* not the memory it points to
--
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