Re: [PATCH v4 02/12] mm/memfd: Introduce MFD_INACCESSIBLE flag

[Chao Peng <chao.p.peng@linux.intel.com>]

On Fri, Jan 21, 2022 at 03:50:55PM +0000, Steven Price wrote:
> On 18/01/2022 13:21, Chao Peng wrote:
> > Introduce a new memfd_create() flag indicating the content of the
> > created memfd is inaccessible from userspace. It does this by force
> > setting F_SEAL_INACCESSIBLE seal when the file is created. It also set
> > F_SEAL_SEAL to prevent future sealing, which means, it can not coexist
> > with MFD_ALLOW_SEALING.
> > 
> > The pages backed by such memfd will be used as guest private memory in
> > confidential computing environments such as Intel TDX/AMD SEV. Since
> > page migration/swapping is not yet supported for such usages so these
> > pages are currently marked as UNMOVABLE and UNEVICTABLE which makes
> > them behave like long-term pinned pages.
> > 
> > Signed-off-by: Chao Peng <chao.p.peng@linux.intel.com>
> > ---
> >  include/uapi/linux/memfd.h |  1 +
> >  mm/memfd.c                 | 20 +++++++++++++++++++-
> >  2 files changed, 20 insertions(+), 1 deletion(-)
> > 
> > diff --git a/include/uapi/linux/memfd.h b/include/uapi/linux/memfd.h
> > index 7a8a26751c23..48750474b904 100644
> > --- a/include/uapi/linux/memfd.h
> > +++ b/include/uapi/linux/memfd.h
> > @@ -8,6 +8,7 @@
> >  #define MFD_CLOEXEC		0x0001U
> >  #define MFD_ALLOW_SEALING	0x0002U
> >  #define MFD_HUGETLB		0x0004U
> > +#define MFD_INACCESSIBLE	0x0008U
> >  
> >  /*
> >   * Huge page size encoding when MFD_HUGETLB is specified, and a huge page
> > diff --git a/mm/memfd.c b/mm/memfd.c
> > index 9f80f162791a..26998d96dc11 100644
> > --- a/mm/memfd.c
> > +++ b/mm/memfd.c
> > @@ -245,16 +245,19 @@ long memfd_fcntl(struct file *file, unsigned int cmd, unsigned long arg)
> >  #define MFD_NAME_PREFIX_LEN (sizeof(MFD_NAME_PREFIX) - 1)
> >  #define MFD_NAME_MAX_LEN (NAME_MAX - MFD_NAME_PREFIX_LEN)
> >  
> > -#define MFD_ALL_FLAGS (MFD_CLOEXEC | MFD_ALLOW_SEALING | MFD_HUGETLB)
> > +#define MFD_ALL_FLAGS (MFD_CLOEXEC | MFD_ALLOW_SEALING | MFD_HUGETLB | \
> > +		       MFD_INACCESSIBLE)
> >  
> >  SYSCALL_DEFINE2(memfd_create,
> >  		const char __user *, uname,
> >  		unsigned int, flags)
> >  {
> > +	struct address_space *mapping;
> >  	unsigned int *file_seals;
> >  	struct file *file;
> >  	int fd, error;
> >  	char *name;
> > +	gfp_t gfp;
> >  	long len;
> >  
> >  	if (!(flags & MFD_HUGETLB)) {
> > @@ -267,6 +270,10 @@ SYSCALL_DEFINE2(memfd_create,
> >  			return -EINVAL;
> >  	}
> >  
> > +	/* Disallow sealing when MFD_INACCESSIBLE is set. */
> > +	if (flags & MFD_INACCESSIBLE && flags & MFD_ALLOW_SEALING)
> > +		return -EINVAL;
> > +
> >  	/* length includes terminating zero */
> >  	len = strnlen_user(uname, MFD_NAME_MAX_LEN + 1);
> >  	if (len <= 0)
> > @@ -315,6 +322,17 @@ SYSCALL_DEFINE2(memfd_create,
> >  		*file_seals &= ~F_SEAL_SEAL;
> >  	}
> >  
> > +	if (flags & MFD_INACCESSIBLE) {
> > +		mapping = file_inode(file)->i_mapping;
> > +		gfp = mapping_gfp_mask(mapping);
> > +		gfp &= ~__GFP_MOVABLE;
> > +		mapping_set_gfp_mask(mapping, gfp);
> > +		mapping_set_unevictable(mapping);
> > +
> > +		file_seals = memfd_file_seals_ptr(file);
> > +		*file_seals &= F_SEAL_SEAL | F_SEAL_INACCESSIBLE;
> 
> This looks backwards - the flags should be set on *file_seals, but here
> you are unsetting all other flags.

Thanks Steve. '|=' actually should be used here.

Chao
> 
> Steve
> 
> > +	}
> > +
> >  	fd_install(fd, file);
> >  	kfree(name);
> >  	return fd;
> >