On Mon, Aug 05, 2024 at 11:34:49AM -0700, Elliot Berman wrote: > This patch was reworked from Patrick's patch: > https://lore.kernel.org/all/20240709132041.3625501-6-roypat@xxxxxxxxxxxx/ > > While guest_memfd is not available to be mapped by userspace, it is > still accessible through the kernel's direct map. This means that in > scenarios where guest-private memory is not hardware protected, it can > be speculatively read and its contents potentially leaked through > hardware side-channels. Removing guest-private memory from the direct > map, thus mitigates a large class of speculative execution issues > [1, Table 1]. > > Direct map removal do not reuse the `.prepare` machinery, since > `prepare` can be called multiple time, and it is the responsibility of > the preparation routine to not "prepare" the same folio twice [2]. Thus, > instead explicitly check if `filemap_grab_folio` allocated a new folio, > and remove the returned folio from the direct map only if this was the > case. > > The patch uses release_folio instead of free_folio to reinsert pages > back into the direct map as by the time free_folio is called, > folio->mapping can already be NULL. This means that a call to > folio_inode inside free_folio might deference a NULL pointer, leaving no > way to access the inode which stores the flags that allow determining > whether the page was removed from the direct map in the first place. > > [1]: https://download.vusec.net/papers/quarantine_raid23.pdf > > Cc: Patrick Roy <roypat@xxxxxxxxxxxx> > Signed-off-by: Elliot Berman <quic_eberman@xxxxxxxxxxx> > --- > include/linux/guest_memfd.h | 8 ++++++ > mm/guest_memfd.c | 65 ++++++++++++++++++++++++++++++++++++++++++++- > 2 files changed, 72 insertions(+), 1 deletion(-) > > diff --git a/include/linux/guest_memfd.h b/include/linux/guest_memfd.h > index be56d9d53067..f9e4a27aed67 100644 > --- a/include/linux/guest_memfd.h > +++ b/include/linux/guest_memfd.h > @@ -25,6 +25,14 @@ struct guest_memfd_operations { > int (*release)(struct inode *inode); > }; > > +/** > + * @GUEST_MEMFD_FLAG_NO_DIRECT_MAP: When making folios inaccessible by host, also > + * remove them from the kernel's direct map. > + */ > +enum { please name this enum, otherwise kernel-doc wont' be happy > + GUEST_MEMFD_FLAG_NO_DIRECT_MAP = BIT(0), > +}; > + > /** > * @GUEST_MEMFD_GRAB_UPTODATE: Ensure pages are zeroed/up to date. > * If trusted hyp will do it, can ommit this flag > diff --git a/mm/guest_memfd.c b/mm/guest_memfd.c > index 580138b0f9d4..e9d8cab72b28 100644 > --- a/mm/guest_memfd.c > +++ b/mm/guest_memfd.c > @@ -7,9 +7,55 @@ > #include <linux/falloc.h> > #include <linux/guest_memfd.h> > #include <linux/pagemap.h> > +#include <linux/set_memory.h> > + > +static inline int guest_memfd_folio_private(struct folio *folio) > +{ > + unsigned long nr_pages = folio_nr_pages(folio); > + unsigned long i; > + int r; > + > + for (i = 0; i < nr_pages; i++) { > + struct page *page = folio_page(folio, i); > + > + r = set_direct_map_invalid_noflush(page); > + if (r < 0) > + goto out_remap; > + } > + > + folio_set_private(folio); > + return 0; > +out_remap: > + for (; i > 0; i--) { > + struct page *page = folio_page(folio, i - 1); > + > + BUG_ON(set_direct_map_default_noflush(page)); > + } > + return r; > +} > + > +static inline void guest_memfd_folio_clear_private(struct folio *folio) > +{ > + unsigned long start = (unsigned long)folio_address(folio); > + unsigned long nr = folio_nr_pages(folio); > + unsigned long i; > + > + if (!folio_test_private(folio)) > + return; > + > + for (i = 0; i < nr; i++) { > + struct page *page = folio_page(folio, i); > + > + BUG_ON(set_direct_map_default_noflush(page)); > + } > + flush_tlb_kernel_range(start, start + folio_size(folio)); I think that TLB flush should come after removing pages from the direct map rather than after adding them back. > + > + folio_clear_private(folio); > +} > > struct folio *guest_memfd_grab_folio(struct file *file, pgoff_t index, u32 flags) > { > + unsigned long gmem_flags = (unsigned long)file->private_data; > struct inode *inode = file_inode(file); > struct guest_memfd_operations *ops = inode->i_private; > struct folio *folio; -- Sincerely yours, Mike.
