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Thu, 22 Apr 2021 16:08:00 +0000 Received: by mail.kernel.org (Postfix) with ESMTPSA id CE755613CB; Thu, 22 Apr 2021 16:07:56 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1619107678; bh=Zs+yUIr4xZ9wfkCTuLnG2sSipMhaTsGYOnAUgTDlM+U=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=BmqcYvbw13vABDK63vVFVscKfkVvrHdoFJlTDsII8KVpAVWDiAKveRZahy9QJJbPy t115tLpQ1W3kraO69UitP+Qp7hhjFRaILPHGgo5DwZjtzWSQTOwpy9OLBTzA+EYd03 86B2+FJ9leSrtuXtvWvpGdIIYVdhp4V6/XKgyOAch6pKn0zLasErLDv74j6QsBbX8/ 3BKg0xqAn2R1Co9XgHfiG8FN0U+W22Gh5xYlpsxLprUHgN6KRibglySm7RigFkcAel imjQJmx9byy7EElPwieEgf8cMwCgmx1tHA4XylD5STxJfURfVjjqNz/lfp6S+/xIuX JMxzdl1MGJysQ== Date: Thu, 22 Apr 2021 17:07:53 +0100 From: Will Deacon To: Mark Rutland Cc: Catalin Marinas , He Zhe , oleg@redhat.com, linux-arm-kernel@lists.infradead.org, paul@paul-moore.com, eparis@redhat.com, linux-audit@redhat.com, linux-kernel@vger.kernel.org Subject: Re: [PATCH 1/3] arm64: ptrace: Add is_syscall_success to handle compat Message-ID: <20210422160752.GA2214@willie-the-truck> References: <20210416075533.7720-1-zhe.he@windriver.com> <20210416123322.GA23184@arm.com> <20210416133431.GA2303@C02TD0UTHF1T.local> <20210419121932.GA30004@willie-the-truck> <20210421171005.GA46949@C02TD0UTHF1T.local> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <20210421171005.GA46949@C02TD0UTHF1T.local> User-Agent: Mutt/1.10.1 (2018-07-13) X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20210422_090759_586995_DC8D4722 X-CRM114-Status: GOOD ( 32.32 ) 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 Hi Mark, On Wed, Apr 21, 2021 at 06:10:05PM +0100, Mark Rutland wrote: > On Mon, Apr 19, 2021 at 01:19:33PM +0100, Will Deacon wrote: > > On Fri, Apr 16, 2021 at 02:34:41PM +0100, Mark Rutland wrote: > > > I think this is a problem we created for ourselves back in commit: > > > > > > 15956689a0e60aa0 ("arm64: compat: Ensure upper 32 bits of x0 are zero on syscall return) > > > > > > AFAICT, the perf regs samples are the only place this matters, since for > > > ptrace the compat regs are implicitly truncated to compat_ulong_t, and > > > audit expects the non-truncated return value. Other architectures don't > > > truncate here, so I think we're setting ourselves up for a game of > > > whack-a-mole to truncate and extend wherever we need to. > > > > > > Given that, I suspect it'd be better to do something like the below. > > > > > > Will, thoughts? > > > > I think perf is one example, but this is also visible to userspace via the > > native ptrace interface and I distinctly remember needing this for some > > versions of arm64 strace to work correctly when tracing compat tasks. > > FWIW, you've convinced me on your approach (more on that below), but > when I went digging here this didn't seem to be exposed via ptrace -- > for any task tracing a compat task, the GPRs are exposed via > compat_gpr_{get,set}(), which always truncate to compat_ulong_t, giving > the lower 32 bits. See task_user_regset_view() for where we get the > regset. > > Am I missing something, or are you thinking of another issue you fixed > at the same time? I think it may depend on whether strace pokes at the GPRs or instead issues a PTRACE_GET_SYSCALL_INFO request but I've forgotten the details, unfortunately. I do remember seeing an issue though, and it was only last year. > > So I do think that clearing the upper bits on the return path is the right > > approach, but it sounds like we need some more work to handle syscall(-1) > > and audit (what exactly is the problem here after these patches have been > > applied?) > > From digging a bit more, I think I agree, and I think these patches are > sufficient for audit. I have some comments I'll leave separately. > > The remaining issues are wherever we assign a signed value to a compat > GPR without explicit truncation. That'll leak via perf sampling the user > regs, but I haven't managed to convince myself whether that causes any > functional change in behaviour for audit, seccomp, or syscall tracing. > > Since we mostly use compat_ulong_t for intermediate values in compat > code, it does look like this is only an issue for x0 where we assign an > error value, e.g. the -ENOSYS case in el0_svc_common. I'll go see if I > can find any more. > > With those fixed up we can remove the x0 truncation from entry.S, > which'd be nice too. If we remove that then we should probably have a (debug?) check on the return-to-user path just to make sure. Will _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel