On Wed, Oct 16, 2024 at 3:16 PM Andrii Nakryiko <andrii@xxxxxxxxxx> wrote: > > From memfd_secret(2) manpage: > > The memory areas backing the file created with memfd_secret(2) are > visible only to the processes that have access to the file descriptor. > The memory region is removed from the kernel page tables and only the > page tables of the processes holding the file descriptor map the > corresponding physical memory. (Thus, the pages in the region can't be > accessed by the kernel itself, so that, for example, pointers to the > region can't be passed to system calls.) > > So folios backed by such secretmem files are not mapped into kernel > address space and shouldn't be accessed, in general. > > To make this a bit more generic of a fix and prevent regression in the > future for similar special mappings, do a generic check of whether the > folio we got is mapped with kernel_page_present(), as suggested in [1]. > This will handle secretmem, and any future special cases that use > a similar approach. > > Original report and repro can be found in [0]. > > [0] https://lore.kernel.org/bpf/ZwyG8Uro%2FSyTXAni@ly-workstation/ > [1] https://lore.kernel.org/bpf/CAJD7tkbpEMx-eC4A-z8Jm1ikrY_KJVjWO+mhhz1_fni4x+COKw@xxxxxxxxxxxxxx/ > > Reported-by: Yi Lai <yi1.lai@xxxxxxxxx> > Suggested-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx> > Fixes: de3ec364c3c3 ("lib/buildid: add single folio-based file reader abstraction") > Signed-off-by: Andrii Nakryiko <andrii@xxxxxxxxxx> > --- > lib/buildid.c | 5 ++++- > 1 file changed, 4 insertions(+), 1 deletion(-) > > diff --git a/lib/buildid.c b/lib/buildid.c > index 290641d92ac1..90df64fd64c1 100644 > --- a/lib/buildid.c > +++ b/lib/buildid.c > @@ -5,6 +5,7 @@ > #include <linux/elf.h> > #include <linux/kernel.h> > #include <linux/pagemap.h> > +#include <linux/set_memory.h> > > #define BUILD_ID 3 > > @@ -74,7 +75,9 @@ static int freader_get_folio(struct freader *r, loff_t file_off) > filemap_invalidate_unlock_shared(r->file->f_mapping); > } > > - if (IS_ERR(r->folio) || !folio_test_uptodate(r->folio)) { > + if (IS_ERR(r->folio) || > + !kernel_page_present(&r->folio->page) || > + !folio_test_uptodate(r->folio)) { Do we need a comment here about the kernel_page_present() check to make it clear that it is handling things like secretmem? > if (!IS_ERR(r->folio)) > folio_put(r->folio); > r->folio = NULL; > -- > 2.43.5 >
