Re: [RFC PATCH RESEND 06/28] mm: mark VMA as locked whenever vma->vm_flags are modified

[Laurent Dufour]

Le 01/09/2022 à 19:34, Suren Baghdasaryan a écrit :
> VMA flag modifications should be done under VMA lock to prevent concurrent
> page fault handling in that area.
> 
> Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> ---
>  fs/proc/task_mmu.c | 1 +
>  fs/userfaultfd.c   | 6 ++++++
>  mm/madvise.c       | 1 +
>  mm/mlock.c         | 2 ++
>  mm/mmap.c          | 1 +
>  mm/mprotect.c      | 1 +
>  6 files changed, 12 insertions(+)

There are few changes also done in the driver's space, for instance:

*** arch/x86/kernel/cpu/sgx/driver.c:
sgx_mmap[98]                   vma->vm_flags |= VM_PFNMAP | VM_DONTEXPAND |
VM_DONTDUMP | VM_IO;
*** arch/x86/kernel/cpu/sgx/virt.c:
sgx_vepc_mmap[108]             vma->vm_flags |= VM_PFNMAP | VM_IO |
VM_DONTDUMP | VM_DONTCOPY;
*** drivers/dax/device.c:
dax_mmap[311]                  vma->vm_flags |= VM_HUGEPAGE;

I guess these changes to vm_flags should be protected as well, or to be
checked one by one.

> 
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index 4e0023643f8b..ceffa5c2c650 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -1285,6 +1285,7 @@ static ssize_t clear_refs_write(struct file *file, const char __user *buf,
>  			for (vma = mm->mmap; vma; vma = vma->vm_next) {
>  				if (!(vma->vm_flags & VM_SOFTDIRTY))
>  					continue;
> +				vma_mark_locked(vma);
>  				vma->vm_flags &= ~VM_SOFTDIRTY;
>  				vma_set_page_prot(vma);
>  			}
> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> index 175de70e3adf..fe557b3d1c07 100644
> --- a/fs/userfaultfd.c
> +++ b/fs/userfaultfd.c
> @@ -620,6 +620,7 @@ static void userfaultfd_event_wait_completion(struct userfaultfd_ctx *ctx,
>  		mmap_write_lock(mm);
>  		for (vma = mm->mmap; vma; vma = vma->vm_next)
>  			if (vma->vm_userfaultfd_ctx.ctx == release_new_ctx) {
> +				vma_mark_locked(vma);
>  				vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
>  				vma->vm_flags &= ~__VM_UFFD_FLAGS;
>  			}
> @@ -653,6 +654,7 @@ int dup_userfaultfd(struct vm_area_struct *vma, struct list_head *fcs)
>  
>  	octx = vma->vm_userfaultfd_ctx.ctx;
>  	if (!octx || !(octx->features & UFFD_FEATURE_EVENT_FORK)) {
> +		vma_mark_locked(vma);
>  		vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
>  		vma->vm_flags &= ~__VM_UFFD_FLAGS;
>  		return 0;
> @@ -734,6 +736,7 @@ void mremap_userfaultfd_prep(struct vm_area_struct *vma,
>  		atomic_inc(&ctx->mmap_changing);
>  	} else {
>  		/* Drop uffd context if remap feature not enabled */
> +		vma_mark_locked(vma);
>  		vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
>  		vma->vm_flags &= ~__VM_UFFD_FLAGS;
>  	}
> @@ -891,6 +894,7 @@ static int userfaultfd_release(struct inode *inode, struct file *file)
>  			vma = prev;
>  		else
>  			prev = vma;
> +		vma_mark_locked(vma);
>  		vma->vm_flags = new_flags;
>  		vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
>  	}
> @@ -1449,6 +1453,7 @@ static int userfaultfd_register(struct userfaultfd_ctx *ctx,
>  		 * the next vma was merged into the current one and
>  		 * the current one has not been updated yet.
>  		 */
> +		vma_mark_locked(vma);
>  		vma->vm_flags = new_flags;
>  		vma->vm_userfaultfd_ctx.ctx = ctx;
>  
> @@ -1630,6 +1635,7 @@ static int userfaultfd_unregister(struct userfaultfd_ctx *ctx,
>  		 * the next vma was merged into the current one and
>  		 * the current one has not been updated yet.
>  		 */
> +		vma_mark_locked(vma);
>  		vma->vm_flags = new_flags;
>  		vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
>  
> diff --git a/mm/madvise.c b/mm/madvise.c
> index 5f0f0948a50e..a173f0025abd 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -181,6 +181,7 @@ static int madvise_update_vma(struct vm_area_struct *vma,
>  	/*
>  	 * vm_flags is protected by the mmap_lock held in write mode.
>  	 */
> +	vma_mark_locked(vma);
>  	vma->vm_flags = new_flags;
>  	if (!vma->vm_file) {
>  		error = replace_anon_vma_name(vma, anon_name);
> diff --git a/mm/mlock.c b/mm/mlock.c
> index b14e929084cc..f62e1a4d05f2 100644
> --- a/mm/mlock.c
> +++ b/mm/mlock.c
> @@ -380,6 +380,7 @@ static void mlock_vma_pages_range(struct vm_area_struct *vma,
>  	 */
>  	if (newflags & VM_LOCKED)
>  		newflags |= VM_IO;
> +	vma_mark_locked(vma);
>  	WRITE_ONCE(vma->vm_flags, newflags);
>  
>  	lru_add_drain();
> @@ -456,6 +457,7 @@ static int mlock_fixup(struct vm_area_struct *vma, struct vm_area_struct **prev,
>  
>  	if ((newflags & VM_LOCKED) && (oldflags & VM_LOCKED)) {
>  		/* No work to do, and mlocking twice would be wrong */
> +		vma_mark_locked(vma);
>  		vma->vm_flags = newflags;
>  	} else {
>  		mlock_vma_pages_range(vma, start, end, newflags);
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 693e6776be39..f89c9b058105 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -1818,6 +1818,7 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
>  out:
>  	perf_event_mmap(vma);
>  
> +	vma_mark_locked(vma);
>  	vm_stat_account(mm, vm_flags, len >> PAGE_SHIFT);
>  	if (vm_flags & VM_LOCKED) {
>  		if ((vm_flags & VM_SPECIAL) || vma_is_dax(vma) ||

I guess, this doesn't really impact, but the call to vma_mark_locked(vma)
may be done only in the case the vm_flags field is touched.
Something like this:

	vm_stat_account(mm, vm_flags, len >> PAGE_SHIFT);
	if (vm_flags & VM_LOCKED) {
		if ((vm_flags & VM_SPECIAL) || vma_is_dax(vma) ||
					is_vm_hugetlb_page(vma) ||
-					vma == get_gate_vma(current->mm))
+					vma == get_gate_vma(current->mm)) {
+			vma_mark_locked(vma);
			vma->vm_flags &= VM_LOCKED_CLEAR_MASK;
-		else
+		} else
			mm->locked_vm += (len >> PAGE_SHIFT);
	}


> diff --git a/mm/mprotect.c b/mm/mprotect.c
> index bc6bddd156ca..df47fc21b0e4 100644
> --- a/mm/mprotect.c
> +++ b/mm/mprotect.c
> @@ -621,6 +621,7 @@ mprotect_fixup(struct mmu_gather *tlb, struct vm_area_struct *vma,
>  	 * vm_flags and vm_page_prot are protected by the mmap_lock
>  	 * held in write mode.
>  	 */
> +	vma_mark_locked(vma);
>  	vma->vm_flags = newflags;
>  	/*
>  	 * We want to check manually if we can change individual PTEs writable