Re: [PATCH v7 01/14] mm: Add F_SEAL_AUTO_ALLOCATE seal to memfd

Chao Peng
2022-08-29T16:38:09Z

On Fri, Aug 26, 2022 at 04:19:32PM +0100, Fuad Tabba wrote:
> Hi Chao,
>
> On Wed, Jul 6, 2022 at 9:25 AM Chao Peng <chao.p.peng@linux.intel.com> wrote:
> >
> > Normally, a write to unallocated space of a file or the hole of a sparse
> > file automatically causes space allocation, for memfd, this equals to
> > memory allocation. This new seal prevents such automatically allocating,
> > either this is from a direct write() or a write on the previously
> > mmap-ed area. The seal does not prevent fallocate() so an explicit
> > fallocate() can still cause allocating and can be used to reserve
> > memory.
> >
> > This is used to prevent unintentional allocation from userspace on a
> > stray or careless write and any intentional allocation should use an
> > explicit fallocate(). One of the main usecases is to avoid memory double
> > allocation for confidential computing usage where we use two memfds to
> > back guest memory and at a single point only one memfd is alive and we
> > want to prevent memory allocation for the other memfd which may have
> > been mmap-ed previously. More discussion can be found at:
> >
> > https://lkml.org/lkml/2022/6/14/1255
> >
> > Suggested-by: Sean Christopherson <seanjc@google.com>
> > Signed-off-by: Chao Peng <chao.p.peng@linux.intel.com>
> > ---
> > include/uapi/linux/fcntl.h | 1 +
> > mm/memfd.c | 3 ++-
> > mm/shmem.c | 16 ++++++++++++++--
> > 3 files changed, 17 insertions(+), 3 deletions(-)
> >
> > diff --git a/include/uapi/linux/fcntl.h b/include/uapi/linux/fcntl.h
> > index 2f86b2ad6d7e..98bdabc8e309 100644
> > --- a/include/uapi/linux/fcntl.h
> > +++ b/include/uapi/linux/fcntl.h
> > @@ -43,6 +43,7 @@
> > #define F_SEAL_GROW 0x0004 /* prevent file from growing */
> > #define F_SEAL_WRITE 0x0008 /* prevent writes */
> > #define F_SEAL_FUTURE_WRITE 0x0010 /* prevent future writes while mapped */
> > +#define F_SEAL_AUTO_ALLOCATE 0x0020 /* prevent allocation for writes */
>
> I think this should also be added to tools/include/uapi/linux/fcntl.h

Yes, thanks.

Chao
>
> Cheers,
> /fuad
>
>
> > /* (1U << 31) is reserved for signed error codes */
> >
> > /*
> > diff --git a/mm/memfd.c b/mm/memfd.c
> > index 08f5f8304746..2afd898798e4 100644
> > --- a/mm/memfd.c
> > +++ b/mm/memfd.c
> > @@ -150,7 +150,8 @@ static unsigned int *memfd_file_seals_ptr(struct file *file)
> > F_SEAL_SHRINK | \
> > F_SEAL_GROW | \
> > F_SEAL_WRITE | \
> > - F_SEAL_FUTURE_WRITE)
> > + F_SEAL_FUTURE_WRITE | \
> > + F_SEAL_AUTO_ALLOCATE)
> >
> > static int memfd_add_seals(struct file *file, unsigned int seals)
> > {
> > diff --git a/mm/shmem.c b/mm/shmem.c
> > index a6f565308133..6c8aef15a17d 100644
> > --- a/mm/shmem.c
> > +++ b/mm/shmem.c
> > @@ -2051,6 +2051,8 @@ static vm_fault_t shmem_fault(struct vm_fault *vmf)
> > struct vm_area_struct *vma = vmf->vma;
> > struct inode *inode = file_inode(vma->vm_file);
> > gfp_t gfp = mapping_gfp_mask(inode->i_mapping);
> > + struct shmem_inode_info *info = SHMEM_I(inode);
> > + enum sgp_type sgp;
> > int err;
> > vm_fault_t ret = VM_FAULT_LOCKED;
> >
> > @@ -2113,7 +2115,12 @@ static vm_fault_t shmem_fault(struct vm_fault *vmf)
> > spin_unlock(&inode->i_lock);
> > }
> >
> > - err = shmem_getpage_gfp(inode, vmf->pgoff, &vmf->page, SGP_CACHE,
> > + if (unlikely(info->seals & F_SEAL_AUTO_ALLOCATE))
> > + sgp = SGP_NOALLOC;
> > + else
> > + sgp = SGP_CACHE;
> > +
> > + err = shmem_getpage_gfp(inode, vmf->pgoff, &vmf->page, sgp,
> > gfp, vma, vmf, &ret);
> > if (err)
> > return vmf_error(err);
> > @@ -2459,6 +2466,7 @@ shmem_write_begin(struct file *file, struct address_space *mapping,
> > struct inode *inode = mapping->host;
> > struct shmem_inode_info *info = SHMEM_I(inode);
> > pgoff_t index = pos >> PAGE_SHIFT;
> > + enum sgp_type sgp;
> > int ret = 0;
> >
> > /* i_rwsem is held by caller */
> > @@ -2470,7 +2478,11 @@ shmem_write_begin(struct file *file, struct address_space *mapping,
> > return -EPERM;
> > }
> >
> > - ret = shmem_getpage(inode, index, pagep, SGP_WRITE);
> > + if (unlikely(info->seals & F_SEAL_AUTO_ALLOCATE))
> > + sgp = SGP_NOALLOC;
> > + else
> > + sgp = SGP_WRITE;
> > + ret = shmem_getpage(inode, index, pagep, sgp);
> >
> > if (ret)
> > return ret;
> > --
> > 2.25.1
> >