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[2001:44b8:1113:6700:899f:c50f:5647:b1f9]) by smtp.gmail.com with ESMTPSA id g19sm306429pfh.134.2019.12.10.17.43.46 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 10 Dec 2019 17:43:47 -0800 (PST) From: Daniel Axtens To: linuxppc-dev@lists.ozlabs.org Subject: [PATCH] powerpc/fault: kernel can extend a user process's stack Date: Wed, 11 Dec 2019 12:43:37 +1100 Message-Id: <20191211014337.28128-1-dja@axtens.net> X-Mailer: git-send-email 2.20.1 MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Tom Lane , Daniel Black , Daniel Axtens Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" If a process page-faults trying to write beyond the end of its stack, we attempt to grow the stack. However, if the kernel attempts to write beyond the end of a process's stack, it takes a bad fault. This can occur when the kernel is trying to set up a signal frame. Permit the kernel to grow a process's stack. The same general limits as to how and when the stack can be grown apply: the kernel code still passes through expand_stack(), so anything that goes beyond e.g. the rlimit should still be blocked. Link: https://bugzilla.kernel.org/show_bug.cgi?id=205183 Reported-by: Tom Lane Cc: Daniel Black Signed-off-by: Daniel Axtens --- arch/powerpc/mm/fault.c | 10 ++++++++++ 1 file changed, 10 insertions(+) diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c index b5047f9b5dec..00183731ea22 100644 --- a/arch/powerpc/mm/fault.c +++ b/arch/powerpc/mm/fault.c @@ -287,7 +287,17 @@ static bool bad_stack_expansion(struct pt_regs *regs, unsigned long address, if (!res) return !store_updates_sp(inst); *must_retry = true; + } else if ((flags & FAULT_FLAG_WRITE) && + !(flags & FAULT_FLAG_USER)) { + /* + * the kernel can also attempt to write beyond the end + * of a process's stack - for example setting up a + * signal frame. We assume this is valid, subject to + * the checks in expand_stack() later. + */ + return false; } + return true; } return false; -- 2.20.1