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dkim=pass header.d=google.com header.s=20210112 header.b=qkgELbA8; spf=pass (imf21.hostedemail.com: domain of jannh@google.com designates 209.85.166.174 as permitted sender) smtp.mailfrom=jannh@google.com; dmarc=pass (policy=reject) header.from=google.com X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: AA32D1C000E X-Rspam-User: X-Stat-Signature: dhprbtzpkgsum16o8su99wu853a9erbf X-HE-Tag: 1664834084-506185 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Mon, Oct 3, 2022 at 11:36 PM Edgecombe, Rick P wrote: > On Mon, 2022-10-03 at 19:26 +0300, Kirill A . Shutemov wrote: > > On Thu, Sep 29, 2022 at 03:29:07PM -0700, Rick Edgecombe wrote: > > > +/* > > > + * Normally the Dirty bit is used to denote COW memory on x86. But > > > + * in the case of X86_FEATURE_SHSTK, the software COW bit is used, > > > + * since the Dirty=1,Write=0 will result in the memory being > > > treated > > > + * as shaodw stack by the HW. So when creating COW memory, a > > > software > > > + * bit is used _PAGE_BIT_COW. The following functions pte_mkcow() > > > and > > > + * pte_clear_cow() take a PTE marked conventially COW (Dirty=1) > > > and > > > + * transition it to the shadow stack compatible version of COW > > > (Cow=1). > > > + */ > > > + > > > +static inline pte_t pte_mkcow(pte_t pte) > > > +{ > > > + if (!cpu_feature_enabled(X86_FEATURE_SHSTK)) > > > + return pte; > > > + > > > + pte = pte_clear_flags(pte, _PAGE_DIRTY); > > > + return pte_set_flags(pte, _PAGE_COW); > > > +} > > > + > > > +static inline pte_t pte_clear_cow(pte_t pte) > > > +{ > > > + /* > > > + * _PAGE_COW is unnecessary on !X86_FEATURE_SHSTK kernels. > > > + * See the _PAGE_COW definition for more details. > > > + */ > > > + if (!cpu_feature_enabled(X86_FEATURE_SHSTK)) > > > + return pte; > > > + > > > + /* > > > + * PTE is getting copied-on-write, so it will be dirtied > > > + * if writable, or made shadow stack if shadow stack and > > > + * being copied on access. Set they dirty bit for both > > > + * cases. > > > + */ > > > + pte = pte_set_flags(pte, _PAGE_DIRTY); > > > + return pte_clear_flags(pte, _PAGE_COW); > > > +} > > > > These X86_FEATURE_SHSTK checks make me uneasy. Maybe use the > > _PAGE_COW > > logic for all machines with 64-bit entries. It will get you much more > > coverage and more universal rules. > > Yes, I didn't like them either at first. The reasoning originally was > that _PAGE_COW is a bit more work and it might show up for some > benchmark. > > Looking at this again though, it is just a few more operations on > memory that is already getting touched either way. It must be a very > tiny amount of impact if any. I'm fine removing them. Having just one > set of logic around this would make it easier to reason about. > > Dave, any thoughts on this? But the rules wouldn't actually be universal - you'd still have to look at X86_FEATURE_SHSTK in code that wants to figure out whether a PTE is shadow stack (on a newer CPU) or readonly dirty (on an older CPU that can set dirty bits on non-present PTEs), right?