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Peter Anvin" , Arnd Bergmann , Al Viro , linux-kernel@vger.kernel.org, linux-hardening@vger.kernel.org Subject: Re: [PATCH v2] x86/uaccess: Avoid check_object_size() in copy_from_user_nmi() Message-ID: References: <20220919201648.2250764-1-keescook@chromium.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220919201648.2250764-1-keescook@chromium.org> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Sep 19, 2022 at 01:16:48PM -0700, Kees Cook wrote: > The check_object_size() helper under CONFIG_HARDENED_USERCOPY is > designed to skip any checks where the length is known at compile time as > a reasonable heuristic to avoid "likely known-good" cases. However, it can > only do this when the copy_*_user() helpers are, themselves, inline too. > > Using find_vmap_area() requires taking a spinlock. The check_object_size() > helper can call find_vmap_area() when the destination is in vmap memory. > If show_regs() is called in interrupt context, it will attempt a call to > copy_from_user_nmi(), which may call check_object_size() and then > find_vmap_area(). If something in normal context happens to be in the > middle of calling find_vmap_area() (with the spinlock held), the interrupt > handler will hang forever. > > The copy_from_user_nmi() call is actually being called with a fixed-size > length, so check_object_size() should never have been called in > the first place. Given the narrow constraints, just replace the > __copy_from_user_inatomic() call with an open-coded version that calls > only into the sanitizers and not check_object_size(), followed by a call > to raw_copy_from_user(). > > Reported-by: Yu Zhao > Link: https://lore.kernel.org/all/CAOUHufaPshtKrTWOz7T7QFYUNVGFm0JBjvM700Nhf9qEL9b3EQ@mail.gmail.com > Reported-by: dev@der-flo.net > Suggested-by: Andrew Morton > Cc: Matthew Wilcox > Cc: Peter Zijlstra > Cc: Josh Poimboeuf > Cc: Dave Hansen > Cc: x86@kernel.org > Fixes: 0aef499f3172 ("mm/usercopy: Detect vmalloc overruns") > Cc: stable@vger.kernel.org > Signed-off-by: Kees Cook > --- Acked-by: Peter Zijlstra (Intel) > arch/x86/lib/usercopy.c | 3 ++- > 1 file changed, 2 insertions(+), 1 deletion(-) > > diff --git a/arch/x86/lib/usercopy.c b/arch/x86/lib/usercopy.c > index ad0139d25401..d2aff9b176cf 100644 > --- a/arch/x86/lib/usercopy.c > +++ b/arch/x86/lib/usercopy.c > @@ -44,7 +44,8 @@ copy_from_user_nmi(void *to, const void __user *from, unsigned long n) > * called from other contexts. > */ > pagefault_disable(); > - ret = __copy_from_user_inatomic(to, from, n); > + instrument_copy_from_user(to, from, n); > + ret = raw_copy_from_user(to, from, n); > pagefault_enable(); > > return ret; > -- > 2.34.1 >