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Donenfeld" , "Gustavo A. R. Silva" , "Mark Rutland" , "Steven Rostedt" , "Mark Brown" , "Guo Hui" , Manoj.Iyer@arm.com, linux-kernel@vger.kernel.org, linux-hardening@vger.kernel.org, "James Yang" , "Shiyou Huang" Subject: Re: [PATCH 1/1] arm64: syscall: Direct PRNG kstack randomization X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20240307_140308_189810_E7D61B2B X-CRM114-Status: GOOD ( 10.96 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Thu, Mar 7, 2024, at 20:15, Kees Cook wrote: > On Thu, Mar 07, 2024 at 12:10:34PM +0100, Arnd Bergmann wrote: >> There is not even any attempt to use the most random bits of >> the cycle counter, as both the high 22 to 24 bits get masked >> out (to keep the wasted stack space small) and the low 3 or 4 >> bits get ignored because of stack alignment. If there was >> any desire to make it more random, a trivial improvement >> would be: >> >> +++ b/include/linux/randomize_kstack.h >> @@ -80,7 +80,7 @@ DECLARE_PER_CPU(u32, kstack_offset); >> if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, \ >> &randomize_kstack_offset)) { \ >> u32 offset = raw_cpu_read(kstack_offset); \ >> - offset ^= (rand); \ >> + offset = ror32(offset, 5) & (rand); \ > > Shouldn't this stay ^ instead of & ? Yes, my mistake. >> raw_cpu_write(kstack_offset, offset); \ >> } \ >> } while (0) > > But yeah, we should likely make this change regardless. Ok, do you want to send a patch, or should I? Arnd _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel