[RAMEN9610-21208]coredump: fix race condition between mmget_not_zero()/get_task_mm...

commit 04f5866e41fb70690e28397487d8bd8eea7d712a upstream.

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.

Change-Id: Iedaf822707c7bf926f7a7212c4e44d9d60fda348
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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(cherry picked from commit 62fe11624b6fce751e89d033308e66b5b0d9c647)

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index f0c7688f5ba4732d97756c47cdc63fc4f6e440ef..5446d42eb9e3c77c3de65c6fbbee20358eb49150 100644 (file)
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -1223,6 +1223,24 @@ static ssize_t clear_refs_write(struct file *file, const char __user *buf,
                                        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);

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 5b0651d62e9610be3c5eede87df1cbd81aa333a6..0c906986f6f92428a1bcf26e8dd5fea27ba6e90d 100644 (file)
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -627,6 +627,8 @@ static void userfaultfd_event_wait_completion(struct userfaultfd_ctx *ctx,
 
                /* 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;
@@ -867,6 +869,8 @@ static int userfaultfd_release(struct inode *inode, struct file *file)
         * 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();
@@ -890,6 +894,7 @@ static int userfaultfd_release(struct inode *inode, struct file *file)
                vma->vm_flags = new_flags;
                vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
        }
+skip_mm:
        up_write(&mm->mmap_sem);
        mmput(mm);
 wakeup:
@@ -1328,6 +1333,8 @@ static int userfaultfd_register(struct userfaultfd_ctx *ctx,
                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;
@@ -1516,6 +1523,8 @@ static int userfaultfd_unregister(struct userfaultfd_ctx *ctx,
                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;

diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
index 3d49b91b674d75640d9eebdb0bff240434e2a6dd..ef4ae0a545fe9fc06752adf2b1971a357a407ba2 100644 (file)
--- a/include/linux/sched/mm.h
+++ b/include/linux/sched/mm.h
@@ -57,6 +57,27 @@ static inline void mmdrop_async(struct mm_struct *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.

diff --git a/mm/mmap.c b/mm/mmap.c
index a05f201917e255f2b3fe9c31a8d9a243b24340ac..7ecec09dd5e5bf4479cddfc311bcc54fdae71fec 100644 (file)
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -45,6 +45,7 @@
 #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>
@@ -2457,7 +2458,8 @@ find_extend_vma(struct mm_struct *mm, unsigned long addr)
        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);
@@ -2483,6 +2485,9 @@ find_extend_vma(struct mm_struct *mm, unsigned long 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;