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 yuzhao@google.com Link: https://lore.kernel.org/all/CAOUHufaPshtKrTWOz7T7QFYUNVGFm0JBjvM700Nhf9qEL9b... Reported-by: dev@der-flo.net Suggested-by: Andrew Morton akpm@linux-foundation.org Cc: Matthew Wilcox willy@infradead.org Cc: Peter Zijlstra peterz@infradead.org Cc: Josh Poimboeuf jpoimboe@kernel.org Cc: Dave Hansen dave.hansen@linux.intel.com Cc: x86@kernel.org Fixes: 0aef499f3172 ("mm/usercopy: Detect vmalloc overruns") Cc: stable@vger.kernel.org Signed-off-by: Kees Cook keescook@chromium.org

---

v2: drop the call explicitly instead of using inline to do it v1: https://lore.kernel.org/lkml/20220916135953.1320601-1-keescook@chromium.org

---

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

Thanks!

Tested-by: Florian Lehner dev@der-flo.net

On Mon, Sep 19, 2022 at 2:16 PM Kees Cook keescook@chromium.org 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 yuzhao@google.com Link: https://lore.kernel.org/all/CAOUHufaPshtKrTWOz7T7QFYUNVGFm0JBjvM700Nhf9qEL9b... Reported-by: dev@der-flo.net Suggested-by: Andrew Morton akpm@linux-foundation.org Cc: Matthew Wilcox willy@infradead.org Cc: Peter Zijlstra peterz@infradead.org Cc: Josh Poimboeuf jpoimboe@kernel.org Cc: Dave Hansen dave.hansen@linux.intel.com Cc: x86@kernel.org Fixes: 0aef499f3172 ("mm/usercopy: Detect vmalloc overruns") Cc: stable@vger.kernel.org Signed-off-by: Kees Cook keescook@chromium.org

---

v2: drop the call explicitly instead of using inline to do it v1: https://lore.kernel.org/lkml/20220916135953.1320601-1-keescook@chromium.org

---

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);
  

Got a build error on top of mm-unstable:

arch/x86/lib/usercopy.c:47:2: error: call to undeclared function 'instrument_copy_from_user'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration] instrument_copy_from_user(to, from, n); ^ arch/x86/lib/usercopy.c:47:2: note: did you mean 'instrument_copy_to_user'? ./include/linux/instrumented.h:117:1: note: 'instrument_copy_to_user' declared here instrument_copy_to_user(void __user *to, const void *from, unsigned long n) ^ 1 error generated.