From bfd06f874fa88be2273de97612784db198100983 Mon Sep 17 00:00:00 2001 From: Linus Torvalds Date: Mon, 8 Apr 2024 20:04:58 -0700 Subject: x86: support user address masking instead of non-speculative conditional commit 2865baf54077aa98fcdb478cefe6a42c417b9374 upstream. The Spectre-v1 mitigations made "access_ok()" much more expensive, since it has to serialize execution with the test for a valid user address. All the normal user copy routines avoid this by just masking the user address with a data-dependent mask instead, but the fast "unsafe_user_read()" kind of patterms that were supposed to be a fast case got slowed down. This introduces a notion of using src = masked_user_access_begin(src); to do the user address sanity using a data-dependent mask instead of the more traditional conditional if (user_read_access_begin(src, len)) { model. This model only works for dense accesses that start at 'src' and on architectures that have a guard region that is guaranteed to fault in between the user space and the kernel space area. With this, the user access doesn't need to be manually checked, because a bad address is guaranteed to fault (by some architecture masking trick: on x86-64 this involves just turning an invalid user address into all ones, since we don't map the top of address space). This only converts a couple of examples for now. Example x86-64 code generation for loading two words from user space: stac mov %rax,%rcx sar $0x3f,%rcx or %rax,%rcx mov (%rcx),%r13 mov 0x8(%rcx),%r14 clac where all the error handling and -EFAULT is now purely handled out of line by the exception path. Of course, if the micro-architecture does badly at 'clac' and 'stac', the above is still pitifully slow. But at least we did as well as we could. Signed-off-by: Linus Torvalds Signed-off-by: Greg Kroah-Hartman --- fs/select.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-) (limited to 'fs') diff --git a/fs/select.c b/fs/select.c index 9515c3fa1a03..bc185d111436 100644 --- a/fs/select.c +++ b/fs/select.c @@ -780,7 +780,9 @@ static inline int get_sigset_argpack(struct sigset_argpack *to, { // the path is hot enough for overhead of copy_from_user() to matter if (from) { - if (!user_read_access_begin(from, sizeof(*from))) + if (can_do_masked_user_access()) + from = masked_user_access_begin(from); + else if (!user_read_access_begin(from, sizeof(*from))) return -EFAULT; unsafe_get_user(to->p, &from->p, Efault); unsafe_get_user(to->size, &from->size, Efault); -- cgit v1.2.3