[PATCH 4.9] hugetlbfs: flush TLBs correctly after huge_pmd_unshare

[Nadav Amit <nadav.amit@gmail.com>]

Below is a patch to address CVE-2021-4002 [1] that I created to backport
to 4.9. The stable kernels of 4.14 and prior ones do not have unified
TLB flushing code, and I managed to mess up the arch code a couple of
times.

Now that the CVE is public, I would appreciate your review of this
patch. I send 4.9 for review - the other ones (4.14 and prior) are
pretty similar.

[1] https://www.openwall.com/lists/oss-security/2021/11/25/1

Thanks,
Nadav

-- >8 --

From: Nadav Amit <namit@vmware.com>
Date: Sat, 20 Nov 2021 12:55:21 -0800
Subject: [kernel v4.9 ] hugetlbfs: flush TLBs correctly after
 huge_pmd_unshare

When __unmap_hugepage_range() calls to huge_pmd_unshare() succeed, a TLB
flush is missing. This TLB flush must be performed before releasing the
i_mmap_rwsem, in order to prevent an unshared PMDs page from being
released and reused before the TLB flush took place.

Arguably, a comprehensive solution would use mmu_gather interface to
batch the TLB flushes and the PMDs page release, however it is not an
easy solution: (1) try_to_unmap_one() and try_to_migrate_one() also call
huge_pmd_unshare() and they cannot use the mmu_gather interface; and (2)
deferring the release of the page reference for the PMDs page until
after i_mmap_rwsem is dropeed can confuse huge_pmd_unshare() into
thinking PMDs are shared when they are not.

Fix __unmap_hugepage_range() by adding the missing TLB flush, and
forcing a flush when unshare is successful.

Fixes: 24669e58477e ("hugetlb: use mmu_gather instead of a temporary linked list for accumulating pages)" # 3.6
Signed-off-by: Nadav Amit <namit@vmware.com>
---
 arch/arm/include/asm/tlb.h  |  8 ++++++++
 arch/ia64/include/asm/tlb.h | 10 ++++++++++
 arch/s390/include/asm/tlb.h | 14 ++++++++++++++
 arch/sh/include/asm/tlb.h   | 10 ++++++++++
 arch/um/include/asm/tlb.h   | 12 ++++++++++++
 include/asm-generic/tlb.h   |  2 ++
 mm/hugetlb.c                | 19 +++++++++++++++++++
 mm/memory.c                 | 16 ++++++++++++++++
 8 files changed, 91 insertions(+)

diff --git a/arch/arm/include/asm/tlb.h b/arch/arm/include/asm/tlb.h
index 1e25cd80589e..1cee2d540956 100644
--- a/arch/arm/include/asm/tlb.h
+++ b/arch/arm/include/asm/tlb.h
@@ -278,6 +278,14 @@ tlb_remove_pmd_tlb_entry(struct mmu_gather *tlb, pmd_t *pmdp, unsigned long addr
 	tlb_add_flush(tlb, addr);
 }
 
+static inline void
+tlb_flush_pmd_range(struct mmu_gather *tlb, unsigned long address,
+		    unsigned long size)
+{
+	tlb_add_flush(tlb, address);
+	tlb_add_flush(tlb, address + size - PMD_SIZE);
+}
+
 #define pte_free_tlb(tlb, ptep, addr)	__pte_free_tlb(tlb, ptep, addr)
 #define pmd_free_tlb(tlb, pmdp, addr)	__pmd_free_tlb(tlb, pmdp, addr)
 #define pud_free_tlb(tlb, pudp, addr)	pud_free((tlb)->mm, pudp)
diff --git a/arch/ia64/include/asm/tlb.h b/arch/ia64/include/asm/tlb.h
index 77e541cf0e5d..34f4a5359561 100644
--- a/arch/ia64/include/asm/tlb.h
+++ b/arch/ia64/include/asm/tlb.h
@@ -272,6 +272,16 @@ __tlb_remove_tlb_entry (struct mmu_gather *tlb, pte_t *ptep, unsigned long addre
 	tlb->end_addr = address + PAGE_SIZE;
 }
 
+static inline void
+tlb_flush_pmd_range(struct mmu_gather *tlb, unsigned long address,
+		    unsigned long size)
+{
+	if (tlb->start_addr > address)
+		tlb->start_addr = address;
+	if (tlb->end_addr < address + size)
+		tlb->end_addr = address + size;
+}
+
 #define tlb_migrate_finish(mm)	platform_tlb_migrate_finish(mm)
 
 #define tlb_start_vma(tlb, vma)			do { } while (0)
diff --git a/arch/s390/include/asm/tlb.h b/arch/s390/include/asm/tlb.h
index 15711de10403..d2681d5a3d5a 100644
--- a/arch/s390/include/asm/tlb.h
+++ b/arch/s390/include/asm/tlb.h
@@ -116,6 +116,20 @@ static inline void tlb_remove_page_size(struct mmu_gather *tlb,
 	return tlb_remove_page(tlb, page);
 }
 
+static inline void tlb_flush_pmd_range(struct mmu_gather *tlb,
+				unsigned long address, unsigned long size)
+{
+	/*
+	 * the range might exceed the original range that was provided to
+	 * tlb_gather_mmu(), so we need to update it despite the fact it is
+	 * usually not updated.
+	 */
+	if (tlb->start > address)
+		tlb->start = address;
+	if (tlb->end < address + size)
+		tlb->end = address + size;
+}
+
 /*
  * pte_free_tlb frees a pte table and clears the CRSTE for the
  * page table from the tlb.
diff --git a/arch/sh/include/asm/tlb.h b/arch/sh/include/asm/tlb.h
index 025cdb1032f6..7aba716fd9a5 100644
--- a/arch/sh/include/asm/tlb.h
+++ b/arch/sh/include/asm/tlb.h
@@ -115,6 +115,16 @@ static inline bool __tlb_remove_page_size(struct mmu_gather *tlb,
 	return __tlb_remove_page(tlb, page);
 }
 
+static inline voide
+tlb_flush_pmd_range(struct mmu_gather *tlb, unsigned long address,
+		    unsigned long size)
+{
+	if (tlb->start > address)
+		tlb->start = address;
+	if (tlb->end < address + size)
+		tlb->end = address + size;
+}
+
 static inline bool __tlb_remove_pte_page(struct mmu_gather *tlb,
 					 struct page *page)
 {
diff --git a/arch/um/include/asm/tlb.h b/arch/um/include/asm/tlb.h
index 821ff0acfe17..6fb47b17179f 100644
--- a/arch/um/include/asm/tlb.h
+++ b/arch/um/include/asm/tlb.h
@@ -128,6 +128,18 @@ static inline void tlb_remove_page_size(struct mmu_gather *tlb,
 	return tlb_remove_page(tlb, page);
 }
 
+static inline void
+tlb_flush_pmd_range(struct mmu_gather *tlb, unsigned long address,
+		    unsigned long size)
+{
+	tlb->need_flush = 1;
+
+	if (tlb->start > address)
+		tlb->start = address;
+	if (tlb->end < address + size)
+		tlb->end = address + size;
+}
+
 /**
  * tlb_remove_tlb_entry - remember a pte unmapping for later tlb invalidation.
  *
diff --git a/include/asm-generic/tlb.h b/include/asm-generic/tlb.h
index c6d667187608..e9851100c0f7 100644
--- a/include/asm-generic/tlb.h
+++ b/include/asm-generic/tlb.h
@@ -123,6 +123,8 @@ void tlb_finish_mmu(struct mmu_gather *tlb, unsigned long start,
 							unsigned long end);
 extern bool __tlb_remove_page_size(struct mmu_gather *tlb, struct page *page,
 				   int page_size);
+void tlb_flush_pmd_range(struct mmu_gather *tlb, unsigned long address,
+			 unsigned long size);
 
 static inline void __tlb_adjust_range(struct mmu_gather *tlb,
 				      unsigned long address)
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index de89e9295f6c..7d51211995b9 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -3395,6 +3395,7 @@ void __unmap_hugepage_range(struct mmu_gather *tlb, struct vm_area_struct *vma,
 	unsigned long sz = huge_page_size(h);
 	const unsigned long mmun_start = start;	/* For mmu_notifiers */
 	const unsigned long mmun_end   = end;	/* For mmu_notifiers */
+	bool force_flush = false;
 
 	WARN_ON(!is_vm_hugetlb_page(vma));
 	BUG_ON(start & ~huge_page_mask(h));
@@ -3411,6 +3412,8 @@ void __unmap_hugepage_range(struct mmu_gather *tlb, struct vm_area_struct *vma,
 		ptl = huge_pte_lock(h, mm, ptep);
 		if (huge_pmd_unshare(mm, &address, ptep)) {
 			spin_unlock(ptl);
+			tlb_flush_pmd_range(tlb, address & PUD_MASK, PUD_SIZE);
+			force_flush = true;
 			continue;
 		}
 
@@ -3467,6 +3470,22 @@ void __unmap_hugepage_range(struct mmu_gather *tlb, struct vm_area_struct *vma,
 	}
 	mmu_notifier_invalidate_range_end(mm, mmun_start, mmun_end);
 	tlb_end_vma(tlb, vma);
+
+	/*
+	 * If we unshared PMDs, the TLB flush was not recorded in mmu_gather. We
+	 * could defer the flush until now, since by holding i_mmap_rwsem we
+	 * guaranteed that the last refernece would not be dropped. But we must
+	 * do the flushing before we return, as otherwise i_mmap_rwsem will be
+	 * dropped and the last reference to the shared PMDs page might be
+	 * dropped as well.
+	 *
+	 * In theory we could defer the freeing of the PMD pages as well, but
+	 * huge_pmd_unshare() relies on the exact page_count for the PMD page to
+	 * detect sharing, so we cannot defer the release of the page either.
+	 * Instead, do flush now.
+	 */
+	if (force_flush)
+		tlb_flush_mmu(tlb);
 }
 
 void __unmap_hugepage_range_final(struct mmu_gather *tlb,
diff --git a/mm/memory.c b/mm/memory.c
index be592d434ad8..c2890dc104d9 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -320,6 +320,22 @@ bool __tlb_remove_page_size(struct mmu_gather *tlb, struct page *page, int page_
 	return false;
 }
 
+void tlb_flush_pmd_range(struct mmu_gather *tlb, unsigned long address,
+			 unsigned long size)
+{
+	if (tlb->page_size != 0 && tlb->page_size != PMD_SIZE)
+		tlb_flush_mmu(tlb);
+
+	tlb->page_size = PMD_SIZE;
+	tlb->start = min(tlb->start, address);
+	tlb->end = max(tlb->end, address + size);
+	/*
+	 * Track the last address with which we adjusted the range. This
+	 * will be used later to adjust again after a mmu_flush due to
+	 * failed __tlb_remove_page
+	 */
+	tlb->addr = address + size - PMD_SIZE;
+}
 #endif /* HAVE_GENERIC_MMU_GATHER */
 
 #ifdef CONFIG_HAVE_RCU_TABLE_FREE
-- 
2.25.1