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Received: from fangorn.home.surriel.com ([10.0.13.7]) by shelob.surriel.com with esmtpsa (TLS1.2) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.97.1) (envelope-from ) id 1wPiPM-0000000024Q-3tUc; Wed, 20 May 2026 11:00:28 -0400 From: Rik van Riel To: linux-kernel@vger.kernel.org Cc: kernel-team@meta.com, linux-mm@kvack.org, david@kernel.org, willy@infradead.org, surenb@google.com, hannes@cmpxchg.org, ljs@kernel.org, ziy@nvidia.com, usama.arif@linux.dev, fvdl@google.com, Rik van Riel Subject: [RFC PATCH 32/40] mm: debug: prevent infinite recursion in dump_page() with CMA Date: Wed, 20 May 2026 10:59:38 -0400 Message-ID: <20260520150018.2491267-33-riel@surriel.com> X-Mailer: git-send-email 2.54.0 In-Reply-To: <20260520150018.2491267-1-riel@surriel.com> References: <20260520150018.2491267-1-riel@surriel.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspam-User: X-Rspamd-Queue-Id: 17B6C180004 X-Rspamd-Server: rspam04 X-Stat-Signature: 695qdnsyjabektpyna6i51djaihr98yg X-HE-Tag: 1779289262-234504 X-HE-Meta: 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 5NIP7uvB QJZGM3MKPJIOaLbw2xPWEvYegco+nBP5Ci4UGDfL9QTJdmnzPf1jLGJh5YPGmf54KMNFaxKGSUeViTs7q3nkM8XnM8X8LJ+T5RjA/f+a7BVcGCwNwCt+ZkUbjuNaoATLYPgAh6P9vFNIa6K7QnhNdlUyeSJMrowVO3mYEDeHgGagowClx1KkqMQ/CtW5Zn+iASCidhQxv4wM6JgzhtRqEdhP9sn7BaxWQu7FvEVDYKYeSL+ljd/+1NihjvfQoF/8LiDvIJWrn/U/VebHPFidGtnV+3vXgPhrfJjsZj67Fh+qWjMH0bxrUj6kdB3pOGf221/mL Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: dump_page() calls is_migrate_cma_folio() which expands to get_pfnblock_migratetype(&folio->page, pfn). That helper resolves the pageblock via pfn_to_pageblock(), and on !CONFIG_SPARSEMEM configurations pfn_to_pageblock() reads page_zone(page) to compute the per-zone pageblock_data offset. When dump_page() is invoked on a page whose zone is not initialised (unavailable PFN ranges, very early boot, or a poisoned struct page), the page_zone() dereference returns garbage and a downstream VM_BUG_ON_PAGE in dump_page()'s own consistency checks fires. The BUG handler then calls dump_page() on the same page, which re-enters the same code path, hits the same BUG, and recurses until the kernel runs out of stack. Guard the is_migrate_cma_folio() call with pfn_valid() and only resolve page_zone() once that has succeeded; only then run zone_spans_pfn() before classifying the page. dump_page() can now safely report on pages without a meaningful zone, and the "CMA" suffix is only printed if the page is genuinely in a CMA pageblock. Found by: dump_page() called from a VM_BUG_ON_PAGE in early boot hitting a page in an unavailable range, recursing until stack exhaustion. Signed-off-by: Rik van Riel Assisted-by: Claude:claude-opus-4.7 syzkaller --- mm/debug.c | 18 +++++++++++++++++- 1 file changed, 17 insertions(+), 1 deletion(-) diff --git a/mm/debug.c b/mm/debug.c index d4542d5d202b..e233520b009c 100644 --- a/mm/debug.c +++ b/mm/debug.c @@ -73,6 +73,7 @@ static void __dump_folio(const struct folio *folio, const struct page *page, { struct address_space *mapping = folio_mapping(folio); int mapcount = atomic_read(&page->_mapcount) + 1; + bool cma = false; char *type = ""; if (page_mapcount_is_type(mapcount)) @@ -112,9 +113,24 @@ static void __dump_folio(const struct folio *folio, const struct page *page, * "isolate" again in the meantime, but since we are just dumping the * state for debugging, it should be fine to accept a bit of * inaccuracy here due to racing. + * + * Guard the is_migrate_cma_folio() call with pfn_valid() and + * zone_spans_pfn(). The macro calls get_pfnblock_migratetype() + * which calls get_pfnblock_flags_word() which has a VM_BUG_ON_PAGE + * for !zone_spans_pfn(). If that fires, dump_page() recurses + * infinitely. Call page_zone() only after pfn_valid() to avoid + * dereferencing uninitialized zone data during early boot. */ +#ifdef CONFIG_CMA + if (pfn_valid(pfn)) { + struct zone *zone = page_zone(page); + + if (zone_spans_pfn(zone, pfn)) + cma = is_migrate_cma_folio(folio, pfn); + } +#endif pr_warn("%sflags: %pGp%s\n", type, &folio->flags, - is_migrate_cma_folio(folio, pfn) ? " CMA" : ""); + cma ? " CMA" : ""); if (page_has_type(&folio->page)) pr_warn("page_type: %x(%s)\n", folio->page.page_type >> 24, page_type_name(folio->page.page_type)); -- 2.54.0