6.6-stable review patch. If anyone has any objections, please let me know.
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From: xu xin xu.xin16@zte.com.cn
[ Upstream commit 590c03ca6a3fbb114396673314e2aa483839608b ]
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 [ changed mm_flags_test() and mm_flags_clear() calls to test_bit() and clear_bit() ] Signed-off-by: Sasha Levin sashal@kernel.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- mm/ksm.c | 18 ++++++++++++++++-- 1 file changed, 16 insertions(+), 2 deletions(-)
--- a/mm/ksm.c +++ b/mm/ksm.c @@ -2451,8 +2451,14 @@ 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. + */ clear_bit(MMF_VM_MERGEABLE, &mm->flags); - clear_bit(MMF_VM_MERGE_ANY, &mm->flags); mmap_read_unlock(mm); mmdrop(mm); } else { @@ -2567,8 +2573,16 @@ void ksm_add_vma(struct vm_area_struct * { struct mm_struct *mm = vma->vm_mm;
- if (test_bit(MMF_VM_MERGE_ANY, &mm->flags)) + if (test_bit(MMF_VM_MERGE_ANY, &mm->flags)) { __ksm_add_vma(vma); + /* + * 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(!test_bit(MMF_VM_MERGEABLE, &mm->flags))) + __ksm_enter(mm); + } }
static void ksm_add_vmas(struct mm_struct *mm)