CVE-2019-11599: Improper Locking (CWE-667)
value: The coredump implementation in the Linux kernel before 5.0.10 does not use locking or other mechanisms to prevent vma layout or vma flags changes while it runs, which allows local users to obtain sensitive information, cause a denial of service, or possibly have unspecified other impact by triggering a race condition with mmget_not_zero or get_task_mm calls. This is related to fs/userfaultfd.c, mm/mmap.c, fs/proc/task_mmu.c, and drivers/infiniband/core/uverbs_main.c.
CWE description: The product does not properly acquire or release a lock on a resource, leading to unexpected resource state changes and behaviors.
CVSS v3 base score: 7
Published: 2019-04-29T18:29Z
Repository: https://github.com/torvalds/linux
Language: C
Files changed: drivers/infiniband/core/uverbs_main.c, fs/proc/task_mmu.c, fs/userfaultfd.c, include/linux/sched/mm.h, mm/mmap.c
Commit date: 2019-04-19 09:46:05 -0700
Commit message: coredump: fix race condition between mmget_not_zero()/get_task_mm() and core dumping

The core dumping code has always run without holding the mmap_sem for
writing, despite that is the only way to ensure that the entire vma
layout will not change from under it.  Only using some signal
serialization on the processes belonging to the mm is not nearly enough.
This was pointed out earlier.  For example in Hugh's post from Jul 2017:

  https://lkml.kernel.org/r/alpine.LSU.2.11.1707191716030.2055@eggly.anvils

  "Not strictly relevant here, but a related note: I was very surprised
   to discover, only quite recently, how handle_mm_fault() may be called
   without down_read(mmap_sem) - when core dumping. That seems a
   misguided optimization to me, which would also be nice to correct"

In particular because the growsdown and growsup can move the
vm_start/vm_end the various loops the core dump does around the vma will
not be consistent if page faults can happen concurrently.

Pretty much all users calling mmget_not_zero()/get_task_mm() and then
taking the mmap_sem had the potential to introduce unexpected side
effects in the core dumping code.

Adding mmap_sem for writing around the ->core_dump invocation is a
viable long term fix, but it requires removing all copy user and page
faults and to replace them with get_dump_page() for all binary formats
which is not suitable as a short term fix.

For the time being this solution manually covers the places that can
confuse the core dump either by altering the vma layout or the vma flags
while it runs.  Once ->core_dump runs under mmap_sem for writing the
function mmget_still_valid() can be dropped.

Allowing mmap_sem protected sections to run in parallel with the
coredump provides some minor parallelism advantage to the swapoff code
(which seems to be safe enough by never mangling any vma field and can
keep doing swapins in parallel to the core dumping) and to some other
corner case.

In order to facilitate the backporting I added "Fixes: 86039bd3b4e6"
however the side effect of this same race condition in /proc/pid/mem
should be reproducible since before 2.6.12-rc2 so I couldn't add any
other "Fixes:" because there's no hash beyond the git genesis commit.

Because find_extend_vma() is the only location outside of the process
context that could modify the "mm" structures under mmap_sem for
reading, by adding the mmget_still_valid() check to it, all other cases
that take the mmap_sem for reading don't need the new check after
mmget_not_zero()/get_task_mm().  The expand_stack() in page fault
context also doesn't need the new check, because all tasks under core
dumping are frozen.

Link: http://lkml.kernel.org/r/20190325224949.11068-1-aarcange@redhat.com
Fixes: 86039bd3b4e6 ("userfaultfd: add new syscall to provide memory externalization")
Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Reported-by: Jann Horn <jannh@google.com>
Suggested-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Mike Rapoport <rppt@linux.ibm.com>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: Jann Horn <jannh@google.com>
Acked-by: Jason Gunthorpe <jgg@mellanox.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Diff: diff --git a/drivers/infiniband/core/uverbs_main.c b/drivers/infiniband/core/uverbs_main.c
index 70b7d80431a9..f2e7ffe6fc54 100644
--- a/drivers/infiniband/core/uverbs_main.c
+++ b/drivers/infiniband/core/uverbs_main.c
@@ -991,10 +991,12 @@ void uverbs_user_mmap_disassociate(struct ib_uverbs_file *ufile)
 		 * the vma_ops callbacks, so we have to clean the list one mm
 		 * at a time to get the lock ordering right. Typically there
 		 * will only be one mm, so no big deal.
 		 */
 		down_write(&mm->mmap_sem);
+		if (!mmget_still_valid(mm))
+			goto skip_mm;
 		mutex_lock(&ufile->umap_lock);
 		list_for_each_entry_safe (priv, next_priv, &ufile->umaps,
 					  list) {
 			struct vm_area_struct *vma = priv->vma;
 
@@ -1005,10 +1007,11 @@ void uverbs_user_mmap_disassociate(struct ib_uverbs_file *ufile)
 			zap_vma_ptes(vma, vma->vm_start,
 				     vma->vm_end - vma->vm_start);
 			vma->vm_flags &= ~(VM_SHARED | VM_MAYSHARE);
 		}
 		mutex_unlock(&ufile->umap_lock);
+	skip_mm:
 		up_write(&mm->mmap_sem);
 		mmput(mm);
 	}
 }
 
diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 92a91e7816d8..95ca1fe7283c 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -1141,10 +1141,28 @@ static ssize_t clear_refs_write(struct file *file, const char __user *buf,
 				up_read(&mm->mmap_sem);
 				if (down_write_killable(&mm->mmap_sem)) {
 					count = -EINTR;
 					goto out_mm;
 				}
+				/*
+				 * Avoid to modify vma->vm_flags
+				 * without locked ops while the
+				 * coredump reads the vm_flags.
+				 */
+				if (!mmget_still_valid(mm)) {
+					/*
+					 * Silently return "count"
+					 * like if get_task_mm()
+					 * failed. FIXME: should this
+					 * function have returned
+					 * -ESRCH if get_task_mm()
+					 * failed like if
+					 * get_proc_task() fails?
+					 */
+					up_write(&mm->mmap_sem);
+					goto out_mm;
+				}
 				for (vma = mm->mmap; vma; vma = vma->vm_next) {
 					vma->vm_flags &= ~VM_SOFTDIRTY;
 					vma_set_page_prot(vma);
 				}
 				downgrade_write(&mm->mmap_sem);
diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 89800fc7dc9d..f5de1e726356 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -627,10 +627,12 @@ static void userfaultfd_event_wait_completion(struct userfaultfd_ctx *ctx,
 		struct vm_area_struct *vma;
 		struct mm_struct *mm = release_new_ctx->mm;
 
 		/* the various vma->vm_userfaultfd_ctx still points to it */
 		down_write(&mm->mmap_sem);
+		/* no task can run (and in turn coredump) yet */
+		VM_WARN_ON(!mmget_still_valid(mm));
 		for (vma = mm->mmap; vma; vma = vma->vm_next)
 			if (vma->vm_userfaultfd_ctx.ctx == release_new_ctx) {
 				vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
 				vma->vm_flags &= ~(VM_UFFD_WP | VM_UFFD_MISSING);
 			}
@@ -881,10 +883,12 @@ static int userfaultfd_release(struct inode *inode, struct file *file)
 	 * changes while handle_userfault released the mmap_sem. So
 	 * it's critical that released is set to true (above), before
 	 * taking the mmap_sem for writing.
 	 */
 	down_write(&mm->mmap_sem);
+	if (!mmget_still_valid(mm))
+		goto skip_mm;
 	prev = NULL;
 	for (vma = mm->mmap; vma; vma = vma->vm_next) {
 		cond_resched();
 		BUG_ON(!!vma->vm_userfaultfd_ctx.ctx ^
 		       !!(vma->vm_flags & (VM_UFFD_MISSING | VM_UFFD_WP)));
@@ -903,10 +907,11 @@ static int userfaultfd_release(struct inode *inode, struct file *file)
 		else
 			prev = vma;
 		vma->vm_flags = new_flags;
 		vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
 	}
+skip_mm:
 	up_write(&mm->mmap_sem);
 	mmput(mm);
 wakeup:
 	/*
 	 * After no new page faults can wait on this fault_*wqh, flush
@@ -1331,10 +1336,12 @@ static int userfaultfd_register(struct userfaultfd_ctx *ctx,
 	ret = -ENOMEM;
 	if (!mmget_not_zero(mm))
 		goto out;
 
 	down_write(&mm->mmap_sem);
+	if (!mmget_still_valid(mm))
+		goto out_unlock;
 	vma = find_vma_prev(mm, start, &prev);
 	if (!vma)
 		goto out_unlock;
 
 	/* check that there's at least one vma in the range */
@@ -1518,10 +1525,12 @@ static int userfaultfd_unregister(struct userfaultfd_ctx *ctx,
 	ret = -ENOMEM;
 	if (!mmget_not_zero(mm))
 		goto out;
 
 	down_write(&mm->mmap_sem);
+	if (!mmget_still_valid(mm))
+		goto out_unlock;
 	vma = find_vma_prev(mm, start, &prev);
 	if (!vma)
 		goto out_unlock;
 
 	/* check that there's at least one vma in the range */
diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
index 0cd9f10423fb..a3fda9f024c3 100644
--- a/include/linux/sched/mm.h
+++ b/include/linux/sched/mm.h
@@ -47,10 +47,31 @@ static inline void mmdrop(struct mm_struct *mm)
 	 */
 	if (unlikely(atomic_dec_and_test(&mm->mm_count)))
 		__mmdrop(mm);
 }
 
+/*
+ * This has to be called after a get_task_mm()/mmget_not_zero()
+ * followed by taking the mmap_sem for writing before modifying the
+ * vmas or anything the coredump pretends not to change from under it.
+ *
+ * NOTE: find_extend_vma() called from GUP context is the only place
+ * that can modify the "mm" (notably the vm_start/end) under mmap_sem
+ * for reading and outside the context of the process, so it is also
+ * the only case that holds the mmap_sem for reading that must call
+ * this function. Generally if the mmap_sem is hold for reading
+ * there's no need of this check after get_task_mm()/mmget_not_zero().
+ *
+ * This function can be obsoleted and the check can be removed, after
+ * the coredump code will hold the mmap_sem for writing before
+ * invoking the ->core_dump methods.
+ */
+static inline bool mmget_still_valid(struct mm_struct *mm)
+{
+	return likely(!mm->core_state);
+}
+
 /**
  * mmget() - Pin the address space associated with a &struct mm_struct.
  * @mm: The address space to pin.
  *
  * Make sure that the address space of the given &struct mm_struct doesn't
diff --git a/mm/mmap.c b/mm/mmap.c
index 41eb48d9b527..bd7b9f293b39 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -43,10 +43,11 @@
 #include <linux/printk.h>
 #include <linux/userfaultfd_k.h>
 #include <linux/moduleparam.h>
 #include <linux/pkeys.h>
 #include <linux/oom.h>
+#include <linux/sched/mm.h>
 
 #include <linux/uaccess.h>
 #include <asm/cacheflush.h>
 #include <asm/tlb.h>
 #include <asm/mmu_context.h>
@@ -2523,11 +2524,12 @@ find_extend_vma(struct mm_struct *mm, unsigned long addr)
 
 	addr &= PAGE_MASK;
 	vma = find_vma_prev(mm, addr, &prev);
 	if (vma && (vma->vm_start <= addr))
 		return vma;
-	if (!prev || expand_stack(prev, addr))
+	/* don't alter vm_end if the coredump is running */
+	if (!prev || !mmget_still_valid(mm) || expand_stack(prev, addr))
 		return NULL;
 	if (prev->vm_flags & VM_LOCKED)
 		populate_vma_page_range(prev, addr, prev->vm_end, NULL);
 	return prev;
 }
@@ -2549,10 +2551,13 @@ find_extend_vma(struct mm_struct *mm, unsigned long addr)
 		return NULL;
 	if (vma->vm_start <= addr)
 		return vma;
 	if (!(vma->vm_flags & VM_GROWSDOWN))
 		return NULL;
+	/* don't alter vm_start if the coredump is running */
+	if (!mmget_still_valid(mm))
+		return NULL;
 	start = vma->vm_start;
 	if (expand_stack(vma, addr))
 		return NULL;
 	if (vma->vm_flags & VM_LOCKED)
 		populate_vma_page_range(vma, addr, start, NULL);

Vulnerable code: 	if (!prev || expand_stack(prev, addr))
Fixed code: 		if (!mmget_still_valid(mm))
			goto skip_mm;
	skip_mm:
				/*
				 * Avoid to modify vma->vm_flags
				 * without locked ops while the
				 * coredump reads the vm_flags.
				 */
				if (!mmget_still_valid(mm)) {
					/*
					 * Silently return "count"
					 * like if get_task_mm()
					 * failed. FIXME: should this
					 * function have returned
					 * -ESRCH if get_task_mm()
					 * failed like if
					 * get_proc_task() fails?
					 */
					up_write(&mm->mmap_sem);
					goto out_mm;
				}
		/* no task can run (and in turn coredump) yet */
		VM_WARN_ON(!mmget_still_valid(mm));
	if (!mmget_still_valid(mm))
		goto skip_mm;
skip_mm:
	if (!mmget_still_valid(mm))
		goto out_unlock;
	if (!mmget_still_valid(mm))
		goto out_unlock;
/*
 * This has to be called after a get_task_mm()/mmget_not_zero()
 * followed by taking the mmap_sem for writing before modifying the
 * vmas or anything the coredump pretends not to change from under it.
 *
 * NOTE: find_extend_vma() called from GUP context is the only place
 * that can modify the "mm" (notably the vm_start/end) under mmap_sem
 * for reading and outside the context of the process, so it is also
 * the only case that holds the mmap_sem for reading that must call
 * this function. Generally if the mmap_sem is hold for reading
 * there's no need of this check after get_task_mm()/mmget_not_zero().
 *
 * This function can be obsoleted and the check can be removed, after
 * the coredump code will hold the mmap_sem for writing before
 * invoking the ->core_dump methods.
 */
static inline bool mmget_still_valid(struct mm_struct *mm)
{
	return likely(!mm->core_state);
}

#include <linux/sched/mm.h>
	/* don't alter vm_end if the coredump is running */
	if (!prev || !mmget_still_valid(mm) || expand_stack(prev, addr))
	/* don't alter vm_start if the coredump is running */
	if (!mmget_still_valid(mm))
		return NULL;