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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 d7597f59d1 ("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 3c6f33b727 ("mm/ksm: support fork/exec for prctl")
supported inheriting the MMF_VM_MERGE_ANY flag when a task calls execve().

Finally, commit 3a9e567ca4 ("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 d7597f59d1 ("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: 3c6f33b727 ("mm/ksm: support fork/exec for prctl")
Fixes: d7597f59d1 ("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>
pull/1354/merge
xu xin 2025-10-07 18:28:21 +08:00 committed by Andrew Morton
parent c6307674ed
commit 590c03ca6a
2 changed files with 19 additions and 5 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

4
include/linux/ksm.h
View File

@ -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;

20
mm/ksm.c
View File

@ -2712,8 +2712,14 @@ 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 {
@ -2831,12 +2837,20 @@ 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;
}