[PATCH for 7.4-rc2+ 2/7] riscv: mm: don't let GUP-fast follow invalidated THP PMDs

[Zi Yan]

pmdp_invalidate() leaves a PMD whose pmd_present() is still true, so the
generic pmd_access_permitted() returns true for an invalidated PMD. Then
GUP-fast can grab a reference on the folio and, as the PMD value doesn't
change until the caller finishes, the recheck after taking the reference
passes too.

Code that invalidates a PMD and then decides, from the folio refcount,
whether it may drop the mapping relies on GUP-fast backing off from an
invalidated PMD, like it does from a cleared one. x86, arm64 and powerpc
already reject invalidated PMDs in pmd_access_permitted().

Provide a riscv pmd_access_permitted() that requires _PAGE_PRESENT, so
invalidated (and PROT_NONE) PMDs are not followed by GUP-fast.

Assisted-by: LLM
Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
To: Paul Walmsley <pjw@xxxxxxxxxx>
To: Palmer Dabbelt <palmer@xxxxxxxxxxx>
To: Albert Ou <aou@xxxxxxxxxxxxxxxxx>
Cc: Alexandre Ghiti <alex@xxxxxxxx>
Cc: Jason Gunthorpe <jgg@xxxxxxxxxx>
Cc: John Hubbard <jhubbard@xxxxxxxxxx>
Cc: Peter Xu <peterx@xxxxxxxxxx>
Cc: linux-riscv@xxxxxxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
---
 arch/riscv/include/asm/pgtable.h | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/arch/riscv/include/asm/pgtable.h b/arch/riscv/include/asm/pgtable.h
index d48f90140841e..66aac56631f0c 100644
--- a/arch/riscv/include/asm/pgtable.h
+++ b/arch/riscv/include/asm/pgtable.h
@@ -816,6 +816,12 @@ static inline int pmd_write(pmd_t pmd)
 	return pte_write(pmd_pte(pmd));
 }
 
+#define pmd_access_permitted pmd_access_permitted
+static inline bool pmd_access_permitted(pmd_t pmd, bool write)
+{
+	return (pmd_val(pmd) & _PAGE_PRESENT) && (!write || pmd_write(pmd));
+}
+
 #define pud_write pud_write
 static inline int pud_write(pud_t pud)
 {

-- 
2.53.0