From: xu xin Date: Tue, 30 Dec 2025 02:49:37 +0000 (-0500) Subject: mm/ksm: fix exec/fork inheritance support for prctl X-Git-Tag: v6.12.64~67 X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=6673d1d818f04ff2f397590e5229a7449b0edf77;p=thirdparty%2Fkernel%2Fstable.git mm/ksm: fix exec/fork inheritance support for prctl [ 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 Cc: Stefan Roesch Cc: David Hildenbrand Cc: Jinjiang Tu Cc: Wang Yaxin Cc: Yang Yang Cc: Signed-off-by: Andrew Morton [ changed mm_flags_test() and mm_flags_clear() calls to test_bit() and clear_bit() ] Signed-off-by: Sasha Levin Signed-off-by: Greg Kroah-Hartman --- diff --git a/mm/ksm.c b/mm/ksm.c index 1601e36a819d3..15f558f69093a 100644 --- a/mm/ksm.c +++ b/mm/ksm.c @@ -2704,8 +2704,14 @@ 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 { @@ -2820,8 +2826,16 @@ void ksm_add_vma(struct vm_area_struct *vma) { 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)