mm: fix pin vs. gup mismatch with gate pages

Linux Kernel / Linux Kernel - Dave Hansen [linux.intel.com] - 4 September 2020 01:36 UTC

Gate pages were missed when converting from get to pin_user_pages(). This can lead to refcount imbalances. This is reliably and quickly reproducible running the x86 selftests when vsyscall=emulate is enabled (the default). Fix by using try_grab_page() with appropriate flags passed.

The long story:

Today, pin_user_pages() and get_user_pages() are similar interfaces for manipulating page reference counts. However, "pins" use a "bias" value and manipulate the actual reference count by 1024 instead of 1 used by plain "gets".

That means that pin_user_pages() must be matched with unpin_user_pages() and can't be mixed with a plain put_user_pages() or put_page().

Enter gate pages, like the vsyscall page. They are pages usually in the kernel image, but which are mapped to userspace. Userspace is allowed access to them, including interfaces using get/pin_user_pages(). The refcount of these kernel pages is manipulated just like a normal user page on the get/pin side so that the put/unpin side can work the same for normal user pages or gate pages.

get_gate_page() uses try_get_page() which only bumps the refcount by 1, not 1024, even if called in the pin_user_pages() path. If someone pins a gate page, this happens:

pin_user_pages() get_gate_page() try_get_page() // bump refcount +1 ... some time later unpin_user_pages() page_ref_sub_and_test(page, 1024))

... and boom, we get a refcount off by 1023. This is reliably and quickly reproducible running the x86 selftests when booted with
vsyscall=emulate (the default). The selftests use ptrace(), but I suspect anything using pin_user_pages() on gate pages could hit this.

To fix it, simply use try_grab_page() instead of try_get_page(), and pass 'gup_flags' in so that FOLL_PIN can be respected.

This bug traces back to the very beginning of the FOLL_PIN support in commit 3faa52c03f44 ("mm/gup: track FOLL_PIN pages"), which showed up in the 5.7 release.

9fa2dd946743 mm: fix pin vs. gup mismatch with gate pages
mm/gup.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

Upstream: git.kernel.org


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