mm/ksm: fix exec/fork inheritance support for prctl

Patch series "ksm: fix exec/fork inheritance", v2.

This series fixes exec/fork inheritance.  See the detailed description of
the issue below.

This patch (of 2):

Background
==========

commit d7597f59d1d33 ("mm: add new api to enable ksm per process")
introduced MMF_VM_MERGE_ANY for mm->flags, and allowed user to set it by
prctl() so that the process's VMAs are forcibly scanned by ksmd.

Subsequently, the 3c6f33b7273a ("mm/ksm: support fork/exec for prctl")
supported inheriting the MMF_VM_MERGE_ANY flag when a task calls execve().

Finally, commit 3a9e567ca45fb ("mm/ksm: fix ksm exec support for prctl")
fixed the issue that ksmd doesn't scan the mm_struct with MMF_VM_MERGE_ANY
by adding the mm_slot to ksm_mm_head in __bprm_mm_init().

Problem
=======

In some extreme scenarios, however, this inheritance of MMF_VM_MERGE_ANY
during exec/fork can fail.  For example, when the scanning frequency of
ksmd is tuned extremely high, a process carrying MMF_VM_MERGE_ANY may
still fail to pass it to the newly exec'd process.  This happens because
ksm_execve() is executed too early in the do_execve flow (prematurely
adding the new mm_struct to the ksm_mm_slot list).

As a result, before do_execve completes, ksmd may have already performed a
scan and found that this new mm_struct has no VM_MERGEABLE VMAs, thus
clearing its MMF_VM_MERGE_ANY flag.  Consequently, when the new program
executes, the flag MMF_VM_MERGE_ANY inheritance missed.

Root reason
===========

commit d7597f59d1d33 ("mm: add new api to enable ksm per process") clear
the flag MMF_VM_MERGE_ANY when ksmd found no VM_MERGEABLE VMAs.

Solution
========

Firstly, Don't clear MMF_VM_MERGE_ANY when ksmd found no VM_MERGEABLE
VMAs, because perhaps their mm_struct has just been added to ksm_mm_slot
list, and its process has not yet officially started running or has not
yet performed mmap/brk to allocate anonymous VMAS.

Secondly, recheck MMF_VM_MERGEABLE again if a process takes
MMF_VM_MERGE_ANY, and create a mm_slot and join it into ksm_scan_list
again.

Link: https://lkml.kernel.org/r/20251007182504440BJgK8VXRHh8TD7IGSUIY4@zte.com.cn
Link: https://lkml.kernel.org/r/20251007182821572h_SoFqYZXEP1mvWI4n9VL@zte.com.cn
Fixes: 3c6f33b7273a ("mm/ksm: support fork/exec for prctl")
Fixes: d7597f59d1d3 ("mm: add new api to enable ksm per process")
Signed-off-by: xu xin <xu.xin16@zte.com.cn>
Cc: Stefan Roesch <shr@devkernel.io>
Cc: David Hildenbrand <david@redhat.com>
Cc: Jinjiang Tu <tujinjiang@huawei.com>
Cc: Wang Yaxin <wang.yaxin@zte.com.cn>
Cc: Yang Yang <yang.yang29@zte.com.cn>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/include/linux/ksm.h b/include/linux/ksm.h
index 067538fc4d5855942fadf7b16cb27ad9de4156d3..c982694c987b968f945360ba8cbec5e11d4c9872 100644 (file)
--- a/include/linux/ksm.h
+++ b/include/linux/ksm.h
@@ -17,7 +17,7 @@
 #ifdef CONFIG_KSM
 int ksm_madvise(struct vm_area_struct *vma, unsigned long start,
                unsigned long end, int advice, vm_flags_t *vm_flags);
-vm_flags_t ksm_vma_flags(const struct mm_struct *mm, const struct file *file,
+vm_flags_t ksm_vma_flags(struct mm_struct *mm, const struct file *file,
                         vm_flags_t vm_flags);
 int ksm_enable_merge_any(struct mm_struct *mm);
 int ksm_disable_merge_any(struct mm_struct *mm);
@@ -103,7 +103,7 @@ bool ksm_process_mergeable(struct mm_struct *mm);
 
 #else  /* !CONFIG_KSM */
 
-static inline vm_flags_t ksm_vma_flags(const struct mm_struct *mm,
+static inline vm_flags_t ksm_vma_flags(struct mm_struct *mm,
                const struct file *file, vm_flags_t vm_flags)
 {
        return vm_flags;

diff --git a/mm/ksm.c b/mm/ksm.c
index 1a5c897b40b191d612f4261453f287901ad51279..3aed0478fdcef3c0e8dcdf549b75daa8c958b2b6 100644 (file)
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -2617,8 +2617,14 @@ no_vmas:
                spin_unlock(&ksm_mmlist_lock);
 
                mm_slot_free(mm_slot_cache, mm_slot);
+               /*
+                * Only clear MMF_VM_MERGEABLE. We must not clear
+                * MMF_VM_MERGE_ANY, because for those MMF_VM_MERGE_ANY process,
+                * perhaps their mm_struct has just been added to ksm_mm_slot
+                * list, and its process has not yet officially started running
+                * or has not yet performed mmap/brk to allocate anonymous VMAS.
+                */
                mm_flags_clear(MMF_VM_MERGEABLE, mm);
-               mm_flags_clear(MMF_VM_MERGE_ANY, mm);
                mmap_read_unlock(mm);
                mmdrop(mm);
        } else {
@@ -2736,12 +2742,20 @@ static int __ksm_del_vma(struct vm_area_struct *vma)
  *
  * Returns: @vm_flags possibly updated to mark mergeable.
  */
-vm_flags_t ksm_vma_flags(const struct mm_struct *mm, const struct file *file,
+vm_flags_t ksm_vma_flags(struct mm_struct *mm, const struct file *file,
                         vm_flags_t vm_flags)
 {
        if (mm_flags_test(MMF_VM_MERGE_ANY, mm) &&
-           __ksm_should_add_vma(file, vm_flags))
+           __ksm_should_add_vma(file, vm_flags)) {
                vm_flags |= VM_MERGEABLE;
+               /*
+                * Generally, the flags here always include MMF_VM_MERGEABLE.
+                * However, in rare cases, this flag may be cleared by ksmd who
+                * scans a cycle without finding any mergeable vma.
+                */
+               if (unlikely(!mm_flags_test(MMF_VM_MERGEABLE, mm)))
+                       __ksm_enter(mm);
+       }
 
        return vm_flags;
 }