[PATCH 5.19-stable] mm/gup: fix FOLL_FORCE COW security issue and remove FOLL_COW

24 Aug 2022

commit 5535be3099717646781ce1540cf725965d680e7b upstream.
Ever since the Dirty COW (CVE-2016-5195) security issue happened, we know
that FOLL_FORCE can be possibly dangerous, especially if there are races
that can be exploited by user space.
Right now, it would be sufficient to have some code that sets a PTE of a
R/O-mapped shared page dirty, in order for it to erroneously become
writable by FOLL_FORCE.  The implications of setting a write-protected PTE
dirty might not be immediately obvious to everyone.
And in fact ever since commit 9ae0f87d009c ("mm/shmem: unconditionally set
pte dirty in mfill_atomic_install_pte"), we can use UFFDIO_CONTINUE to map
a shmem page R/O while marking the pte dirty.  This can be used by
unprivileged user space to modify tmpfs/shmem file content even if the
user does not have write permissions to the file, and to bypass memfd
write sealing -- Dirty COW restricted to tmpfs/shmem (CVE-2022-2590).
To fix such security issues for good, the insight is that we really only
need that fancy retry logic (FOLL_COW) for COW mappings that are not
writable (!VM_WRITE).  And in a COW mapping, we really only broke COW if
we have an exclusive anonymous page mapped.  If we have something else
mapped, or the mapped anonymous page might be shared (!PageAnonExclusive),
we have to trigger a write fault to break COW.  If we don't find an
exclusive anonymous page when we retry, we have to trigger COW breaking
once again because something intervened.
Let's move away from this mandatory-retry + dirty handling and rely on our
PageAnonExclusive() flag for making a similar decision, to use the same
COW logic as in other kernel parts here as well.  In case we stumble over
a PTE in a COW mapping that does not map an exclusive anonymous page, COW
was not properly broken and we have to trigger a fake write-fault to break
COW.
Just like we do in can_change_pte_writable() added via commit 64fe24a3e05e
("mm/mprotect: try avoiding write faults for exclusive anonymous pages
when changing protection") and commit 76aefad628aa ("mm/mprotect: fix
soft-dirty check in can_change_pte_writable()"), take care of softdirty
and uffd-wp manually.
For example, a write() via /proc/self/mem to a uffd-wp-protected range has
to fail instead of silently granting write access and bypassing the
userspace fault handler.  Note that FOLL_FORCE is not only used for debug
access, but also triggered by applications without debug intentions, for
example, when pinning pages via RDMA.
This fixes CVE-2022-2590. Note that only x86_64 and aarch64 are
affected, because only those support CONFIG_HAVE_ARCH_USERFAULTFD_MINOR.
Fortunately, FOLL_COW is no longer required to handle FOLL_FORCE. So
let's just get rid of it.
Thanks to Nadav Amit for pointing out that the pte_dirty() check in
FOLL_FORCE code is problematic and might be exploitable.
Note 1: We don't check for the PTE being dirty because it doesn't matter
    for making a "was COWed" decision anymore, and whoever modifies the
    page has to set the page dirty either way.
Note 2: Kernels before extended uffd-wp support and before
    PageAnonExclusive (< 5.19) can simply revert the problematic
    commit instead and be safe regarding UFFDIO_CONTINUE. A backport to
    v5.19 requires minor adjustments due to lack of
    vma_soft_dirty_enabled().
Link: https://lkml.kernel.org/r/20220809205640.70916-1-david@redhat.com
Fixes: 9ae0f87d009c ("mm/shmem: unconditionally set pte dirty in mfill_atomic_install_pte")
Signed-off-by: David Hildenbrand david@redhat.com
Cc: Greg Kroah-Hartman gregkh@linuxfoundation.org
Cc: Axel Rasmussen axelrasmussen@google.com
Cc: Nadav Amit nadav.amit@gmail.com
Cc: Peter Xu peterx@redhat.com
Cc: Hugh Dickins hughd@google.com
Cc: Andrea Arcangeli aarcange@redhat.com
Cc: Matthew Wilcox willy@infradead.org
Cc: Vlastimil Babka vbabka@suse.cz
Cc: John Hubbard jhubbard@nvidia.com
Cc: Jason Gunthorpe jgg@nvidia.com
Cc: David Laight David.Laight@ACULAB.COM
Cc: stable@vger.kernel.org	[5.16]
Signed-off-by: Andrew Morton akpm@linux-foundation.org
Signed-off-by: David Hildenbrand david@redhat.com
---
 include/linux/mm.h |  1 -
 mm/gup.c           | 69 +++++++++++++++++++++++++++++++---------------
 mm/huge_memory.c   | 65 +++++++++++++++++++++++++++++--------------
 3 files changed, 91 insertions(+), 44 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 7898e29bcfb5..25b8860f47cc 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2939,7 +2939,6 @@ struct page *follow_page(struct vm_area_struct *vma, unsigned long address,
 #define FOLL_MIGRATION	0x400	/* wait for page to replace migration entry */
 #define FOLL_TRIED	0x800	/* a retry, previous pass started an IO */
 #define FOLL_REMOTE	0x2000	/* we are working on non-current tsk/mm */
-#define FOLL_COW	0x4000	/* internal GUP flag */
 #define FOLL_ANON	0x8000	/* don't do file mappings */
 #define FOLL_LONGTERM	0x10000	/* mapping lifetime is indefinite: see below */
 #define FOLL_SPLIT_PMD	0x20000	/* split huge pmd before returning */
diff --git a/mm/gup.c b/mm/gup.c
index e2a39e30756d..d2fd46b50102 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -478,14 +478,43 @@ static int follow_pfn_pte(struct vm_area_struct *vma, unsigned long address,
    return -EEXIST;
 }
-/*
- * FOLL_FORCE can write to even unwritable pte's, but only
- * after we've gone through a COW cycle and they are dirty.
- */
-static inline bool can_follow_write_pte(pte_t pte, unsigned int flags)
+/* FOLL_FORCE can write to even unwritable PTEs in COW mappings. */
+static inline bool can_follow_write_pte(pte_t pte, struct page *page,
+					struct vm_area_struct *vma,
+					unsigned int flags)
 {
-	return pte_write(pte) ||
-		((flags & FOLL_FORCE) && (flags & FOLL_COW) && pte_dirty(pte));
+	/* If the pte is writable, we can write to the page. */
+	if (pte_write(pte))
+		return true;
+
+	/* Maybe FOLL_FORCE is set to override it? */
+	if (!(flags & FOLL_FORCE))
+		return false;
+
+	/* But FOLL_FORCE has no effect on shared mappings */
+	if (vma->vm_flags & (VM_MAYSHARE | VM_SHARED))
+		return false;
+
+	/* ... or read-only private ones */
+	if (!(vma->vm_flags & VM_MAYWRITE))
+		return false;
+
+	/* ... or already writable ones that just need to take a write fault */
+	if (vma->vm_flags & VM_WRITE)
+		return false;
+
+	/*
+	 * See can_change_pte_writable(): we broke COW and could map the page
+	 * writable if we have an exclusive anonymous page ...
+	 */
+	if (!page || !PageAnon(page) || !PageAnonExclusive(page))
+		return false;
+
+	/* ... and a write-fault isn't required for other reasons. */
+	if (IS_ENABLED(CONFIG_MEM_SOFT_DIRTY) &&
+	    !(vma->vm_flags & VM_SOFTDIRTY) && !pte_soft_dirty(pte))
+		return false;
+	return !userfaultfd_pte_wp(vma, pte);
 }
static struct page *follow_page_pte(struct vm_area_struct *vma,
@@ -528,12 +557,19 @@ static struct page *follow_page_pte(struct vm_area_struct *vma,
    }
    if ((flags & FOLL_NUMA) && pte_protnone(pte))
    	goto no_page;
-	if ((flags & FOLL_WRITE) && !can_follow_write_pte(pte, flags)) {
-		pte_unmap_unlock(ptep, ptl);
-		return NULL;
-	}
page = vm_normal_page(vma, address, pte);
+
+	/*
+	 * We only care about anon pages in can_follow_write_pte() and don't
+	 * have to worry about pte_devmap() because they are never anon.
+	 */
+	if ((flags & FOLL_WRITE) &&
+	    !can_follow_write_pte(pte, page, vma, flags)) {
+		page = NULL;
+		goto out;
+	}
+
    if (!page && pte_devmap(pte) && (flags & (FOLL_GET | FOLL_PIN))) {
    	/*
    	 * Only return device mapping pages in the FOLL_GET or FOLL_PIN
@@ -967,17 +1003,6 @@ static int faultin_page(struct vm_area_struct *vma,
    	return -EBUSY;
    }
-	/*
-	 * The VM_FAULT_WRITE bit tells us that do_wp_page has broken COW when
-	 * necessary, even if maybe_mkwrite decided not to set pte_write. We
-	 * can thus safely do subsequent page lookups as if they were reads.
-	 * But only do so when looping for pte_write is futile: in some cases
-	 * userspace may also be wanting to write to the gotten user page,
-	 * which a read fault here might prevent (a readonly page might get
-	 * reCOWed by userspace write).
-	 */
-	if ((ret & VM_FAULT_WRITE) && !(vma->vm_flags & VM_WRITE))
-		*flags |= FOLL_COW;
    return 0;
 }
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 834f288b3769..164d13b62079 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -977,12 +977,6 @@ struct page *follow_devmap_pmd(struct vm_area_struct *vma, unsigned long addr,
assert_spin_locked(pmd_lockptr(mm, pmd));
-	/*
-	 * When we COW a devmap PMD entry, we split it into PTEs, so we should
-	 * not be in this function with `flags & FOLL_COW` set.
-	 */
-	WARN_ONCE(flags & FOLL_COW, "mm: In follow_devmap_pmd with FOLL_COW set");
-
    /* FOLL_GET and FOLL_PIN are mutually exclusive. */
    if (WARN_ON_ONCE((flags & (FOLL_PIN | FOLL_GET)) ==
    		 (FOLL_PIN | FOLL_GET)))
@@ -1348,14 +1342,43 @@ vm_fault_t do_huge_pmd_wp_page(struct vm_fault *vmf)
    return VM_FAULT_FALLBACK;
 }
-/*
- * FOLL_FORCE can write to even unwritable pmd's, but only
- * after we've gone through a COW cycle and they are dirty.
- */
-static inline bool can_follow_write_pmd(pmd_t pmd, unsigned int flags)
+/* FOLL_FORCE can write to even unwritable PMDs in COW mappings. */
+static inline bool can_follow_write_pmd(pmd_t pmd, struct page *page,
+					struct vm_area_struct *vma,
+					unsigned int flags)
 {
-	return pmd_write(pmd) ||
-	       ((flags & FOLL_FORCE) && (flags & FOLL_COW) && pmd_dirty(pmd));
+	/* If the pmd is writable, we can write to the page. */
+	if (pmd_write(pmd))
+		return true;
+
+	/* Maybe FOLL_FORCE is set to override it? */
+	if (!(flags & FOLL_FORCE))
+		return false;
+
+	/* But FOLL_FORCE has no effect on shared mappings */
+	if (vma->vm_flags & (VM_MAYSHARE | VM_SHARED))
+		return false;
+
+	/* ... or read-only private ones */
+	if (!(vma->vm_flags & VM_MAYWRITE))
+		return false;
+
+	/* ... or already writable ones that just need to take a write fault */
+	if (vma->vm_flags & VM_WRITE)
+		return false;
+
+	/*
+	 * See can_change_pte_writable(): we broke COW and could map the page
+	 * writable if we have an exclusive anonymous page ...
+	 */
+	if (!page || !PageAnon(page) || !PageAnonExclusive(page))
+		return false;
+
+	/* ... and a write-fault isn't required for other reasons. */
+	if (IS_ENABLED(CONFIG_MEM_SOFT_DIRTY) &&
+	    !(vma->vm_flags & VM_SOFTDIRTY) && !pmd_soft_dirty(pmd))
+		return false;
+	return !userfaultfd_huge_pmd_wp(vma, pmd);
 }
struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
@@ -1364,12 +1387,16 @@ struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
    			   unsigned int flags)
 {
    struct mm_struct *mm = vma->vm_mm;
-	struct page *page = NULL;
+	struct page *page;
assert_spin_locked(pmd_lockptr(mm, pmd));
-	if (flags & FOLL_WRITE && !can_follow_write_pmd(*pmd, flags))
-		goto out;
+	page = pmd_page(*pmd);
+	VM_BUG_ON_PAGE(!PageHead(page) && !is_zone_device_page(page), page);
+
+	if ((flags & FOLL_WRITE) &&
+	    !can_follow_write_pmd(*pmd, page, vma, flags))
+		return NULL;
/* Avoid dumping huge zero page */
    if ((flags & FOLL_DUMP) && is_huge_zero_pmd(*pmd))
@@ -1377,10 +1404,7 @@ struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
/* Full NUMA hinting faults to serialise migration in fault paths */
    if ((flags & FOLL_NUMA) && pmd_protnone(*pmd))
-		goto out;
-
-	page = pmd_page(*pmd);
-	VM_BUG_ON_PAGE(!PageHead(page) && !is_zone_device_page(page), page);
+		return NULL;
if (!pmd_write(*pmd) && gup_must_unshare(flags, page))
    	return ERR_PTR(-EMLINK);
@@ -1397,7 +1421,6 @@ struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
    page += (addr & ~HPAGE_PMD_MASK) >> PAGE_SHIFT;
    VM_BUG_ON_PAGE(!PageCompound(page) && !is_zone_device_page(page), page);
-out:
    return page;
 }
-- 
2.37.1


    

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[PATCH 5.19-stable] mm/gup: fix FOLL_FORCE COW security issue and remove FOLL_COW