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b=eV/6svohXqoRl4XVoK1J7U2ZjwjErN5URCFh3KdhlIXf9gduiYQcr5pYblYJypRGO bj8hXMQJZvBk6kYaJ7A7+HpLuvdg+kB64RtpCjWJUDnYOQle9mHB3cZ5JKZMqfSXtU tNfJCaoU56uOlgi5u+5NibV0OfMBetQDlhQV1tM9opB7z5yqwjUAUAWtUWiqZCuK7V jYAiRFxMnlITtgasNisHbMzza6WYrFwMOPwQq4JrD0estoZ1tcsWN6SgvxQDZaCBvt 18UqadOd+ef9yT3fQ8ue3YuuPYdej8RftGV4aiNH1x8/H8evLpARasnkd8R/x0l6FP XG1RI9ZzlbDoQ== Message-ID: Date: Tue, 25 Aug 2026 17:51:38 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v6 1/3] mm: make persistent huge zero folio read-only To: Xueyuan Chen , akpm@linux-foundation.org Cc: ljs@kernel.org, usama.arif@linux.dev, catalin.marinas@arm.com, will@kernel.org, linux-arm-kernel@lists.infradead.org, tglx@kernel.org, mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com, x86@kernel.org, hpa@zytor.com, rppt@kernel.org, ryan.roberts@arm.com, ziy@nvidia.com, baohua@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Lance Yang References: <20260730090647.2401252-1-xueyuan.chen21@gmail.com> <20260730090647.2401252-2-xueyuan.chen21@gmail.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Stat-Signature: nm7njkk9m8pukbcpmr47ajw7dyu9e9z8 X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 692E040005 X-Rspam-User: X-HE-Tag: 1787673107-660809 X-HE-Meta: U2FsdGVkX1/U/RzQciqA/U9BDDC9vSJkbAZ/NZybfTPvXp44RraYt3+K0iFDBQGsHXB+gEBiT15FHoa8E3rEbJGDwpSAzWhUueKSxfzo8TXitQnVxZRp/qnAGahgcw3UohibXVIi4P/Gd2820xyQCZWfxE65vKRwy10Y/G4wUFhcL8swqsOLGtI5Az/nea4OjuQrGA8FsfmL9fc2+BSh0EZGoKl6QU8pxfAD3KVxy0PF7KRZHuqZvdMwe980V+0lteYXn1iQSB2zQYX91byDgIOgZA4UDDYjeTail2Q9DSj0Joe1IS75B8ZYZEFnfzxNQXODFPmJrjyEyYWVc6F8X+Ayv0C0OENTEXF1IhiVV5BsKXWXbTMfthHzSYkV4zK7Tkti4pHQQxH3hwDHfGtZJ1hEVWmmPyTzX3KsyNzAYufJZC/rb4oTcJeJdfUO3FssQREwBMun8DGJXG5qj1R9NT7X5wxw4NLPbs4NiHz1XRQTomu7baolUvyX0LRPVZypDhp+US37pdAoHBkPHktjDAKetvaE6MdQfsJAKbDAlyz+tRbunWk8vRDQ6bXWzGxlTuB42WRroXNy8v/rdLjpPU4Sp3ZbXB2tjGuRbiQw609dARJrh7AyhIQB5l8hwSNKmrPPYKptXkGDs9cmvHSzgaISbT6QPFCG0YgjkSpdV/g8R2CDMTb2gpaszzkY3SpYxV39QVocTdHiNt30z+EoXiuY2BSHQUZvXMmPXOc49LMn5Z+dMsDZ2sw30ARjsnRdAGbXQHhUPQwOWaHFv6xIql8v49CD3pLxDbm2ZbgkRVYoGW05XaakQQ9Nx1P1eOdEs5B5VZRg7JgEvUqkgITMH48OOFDctxmlITTJumWJi1G2ylNnL6aIno/Ix1lBj2McRSNZSi4D5pQlCfqCVTM8ZjPQgM9j9Rc41JM8ejEx/Ws53/CPvk9NyCkvwd1HVpJocliLBwWL2Uekfr2kuHS NLJjJ1I3 K6+KhrLBTcjp2Zg4zWYNHn38YcVd/rvxDW7rLO8C61729oQT8moEYbgacUh7Y1H99XD8/lm2YdA/bDC2sCsOFyU2F3IL5aXYcpvUpDy1DhI9w+WP9/XdGpd1AXsyX7GFQngLvc+660XsFRy4cFVM8gDM2ZJihKEpvn0pWXP3pBgWs0ikTMONG2lSf7SLPoin6IOmM+BcZ6U7H8pnQHYX4PY8rvhtKh/zWvmrXAHWxlU8cTNSyh4SLjKJU0yQnJ9J3tFANa6XaLjFEbvbBGeGO964YDc7jkXbd4su9X9sWWUZ/X3HQlV7KeODWaQHwFNtXRBp95oaBoe7O9kLy+bCSjH0K1GKpq2qeg4VTFRFDiMvPpTS4MLZsw1zmjc8jR65G9AjL/9qHCDoOS77fnt6KTgtxn9Iukp/ofWVJSZMFfzPQAeqNPtnukfBRw9wrbv5Fj0JTxy9swZ55ooyzt2YrKrSAJw== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 7/30/26 11:06, Xueyuan Chen wrote: > The persistent huge zero folio is shared globally and should stay zero > after initialization. As Jann Horn pointed out[1], kernel bugs have ended > up writing to pages that were meant to be read-only, including in > security-sensitive cases. Making the persistent huge zero folio read-only > in the direct map turns such writes into faults instead of silent zero-page > corruption. > > Add set_direct_map_ro_noflush() so mm code can make a direct-map range > read-only. Use an address-based signature to match ongoing direct-map > helper work[2], where existing page-based helpers may move the same way. > The helper is direct-map specific and leaves TLB invalidation to its > caller. Architectures without direct-map permission support keep existing > behavior through the generic stub. > > The folio is allocated and zeroed through the writable direct map before > thp_shrinker_init() changes its permissions. thp_shrinker_init() is called > from hugepage_init(), which is registered as a subsys_initcall and runs > after SMP initialization. Stale writable kernel TLB entries may therefore > exist. Flush the direct-map range immediately after the page-table update > so they cannot bypass the read-only mapping. > > GFP_TRANSHUGE includes __GFP_HIGHMEM. On 32-bit systems, the persistent > folio may therefore reside in high memory, where folio_address() returns > NULL. Such a folio has no permanent direct-map mapping to protect, so skip > the permission change and TLB flush. > > Treat the direct-map permission change as best-effort. Architectures that > do not implement the helper keep the existing behavior via the generic > stub. > > Inspired by Jann Horn's read-only zero page work[1] and follow-up > discussion[3] with Yang Shi. > > Link: https://lore.kernel.org/r/20260508-ro-zeropage-v1-1-9808abc20b49@google.com [1] > Link: https://lore.kernel.org/r/0e5b23a6-4895-454a-9dfa-6dc21adc2991@kernel.org [2] > Link: https://lore.kernel.org/r/CAHbLzkrXXe7r3n3jXgDKtwZhRqj=jDx9E6dLOULohnhBguvi9A@mail.gmail.com [3] > Link: https://lore.kernel.org/r/20260727113530.5cd347581a66b3279490a604@linux-foundation.org > Suggested-by: David Hildenbrand > Suggested-by: Usama Arif > Co-developed-by: Lance Yang > Signed-off-by: Lance Yang > Signed-off-by: Xueyuan Chen > --- > include/linux/set_memory.h | 29 +++++++++++++++++++++++++++++ > mm/huge_memory.c | 20 +++++++++++++++++++- > 2 files changed, 48 insertions(+), 1 deletion(-) > > diff --git a/include/linux/set_memory.h b/include/linux/set_memory.h > index 3030d9245f5a..e83ced6a3827 100644 > --- a/include/linux/set_memory.h > +++ b/include/linux/set_memory.h > @@ -40,6 +40,24 @@ static inline int set_direct_map_valid_noflush(struct page *page, > return 0; > } > > +/** > + * set_direct_map_ro_noflush - make a direct-map range read-only > + * @addr: start address in the direct map > + * @nr_pages: number of pages starting at @addr > + * > + * Make the direct-map range starting at @addr read-only without invalidating > + * TLBs. Callers must either ensure that no stale writable translations can > + * be used, or treat the permission change as a best-effort hardening step. > + * > + * Return: 0 on success or when direct-map permission changes are unsupported, > + * or a negative errno on failure. > + */ > +static inline int set_direct_map_ro_noflush(const void *addr, > + unsigned long nr_pages) Two tabs here in MM land. > +{ > + return 0; > +} Nobody checks the return value, so how helpful is it to return it? Do we expect other users to check for the return code? I guess we just try to keep consistency with the other functions? [...] > @@ -932,6 +934,8 @@ static int __init thp_shrinker_init(void) > shrinker_register(deferred_split_shrinker); > > if (IS_ENABLED(CONFIG_PERSISTENT_HUGE_ZERO_FOLIO)) { > + unsigned long addr; > + > /* > * Bump the reference of the huge_zero_folio and do not > * initialize the shrinker. > @@ -940,8 +944,22 @@ static int __init thp_shrinker_init(void) > * that get_huge_zero_folio() will most likely not fail as > * thp_shrinker_init() is invoked early on during boot. > */ > - if (!get_huge_zero_folio()) > + if (!get_huge_zero_folio()) { > pr_warn("Allocating persistent huge zero folio failed\n"); > + return 0; > + } > + > + /* Highmem folios have no permanent direct-map mapping to protect. */ > + if (folio_test_highmem(huge_zero_folio)) > + return 0; > + > + addr = (unsigned long)folio_address(huge_zero_folio); > + /* > + * The folio was zeroed through the writable direct map. Flush > + * after the page-table update to invalidate stale translations. > + */ > + set_direct_map_ro_noflush((void *)addr, HPAGE_PMD_NR); > + flush_tlb_kernel_range(addr, addr + HPAGE_PMD_SIZE); > return 0; > } > The code changed upstream in the meantime. There is now the huge_zero_init() we can hook into instead. -- Cheers, David