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Thu, 07 May 2020 09:19:56 +0100 Date: Thu, 7 May 2020 09:19:53 +0100 From: Marc Zyngier To: Luke Nelson Subject: Re: [RFC PATCH bpf-next 1/3] arm64: insn: Fix two bugs in encoding 32-bit logical immediates Message-ID: <20200507091953.70505638@why> In-Reply-To: <20200507010504.26352-2-luke.r.nels@gmail.com> References: <20200507010504.26352-1-luke.r.nels@gmail.com> <20200507010504.26352-2-luke.r.nels@gmail.com> Organization: Approximate X-Mailer: Claws Mail 3.17.5 (GTK+ 2.24.32; x86_64-pc-linux-gnu) MIME-Version: 1.0 X-SA-Exim-Connect-IP: 62.31.163.78 X-SA-Exim-Rcpt-To: lukenels@cs.washington.edu, bpf@vger.kernel.org, luke.r.nels@gmail.com, xi.wang@gmail.com, catalin.marinas@arm.com, will@kernel.org, daniel@iogearbox.net, ast@kernel.org, zlim.lnx@gmail.com, kafai@fb.com, songliubraving@fb.com, yhs@fb.com, andriin@fb.com, john.fastabend@gmail.com, kpsingh@chromium.org, mark.rutland@arm.com, gregkh@linuxfoundation.org, tglx@linutronix.de, christoffer.dall@linaro.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, netdev@vger.kernel.org, clang-built-linux@googlegroups.com X-SA-Exim-Mail-From: maz@kernel.org X-SA-Exim-Scanned: No (on disco-boy.misterjones.org); SAEximRunCond expanded to false X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20200507_011959_224930_8BDA951E X-CRM114-Status: GOOD ( 33.05 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Mark Rutland , Song Liu , Catalin Marinas , Alexei Starovoitov , Will Deacon , Daniel Borkmann , John Fastabend , clang-built-linux@googlegroups.com, Zi Shen Lim , Yonghong Song , Andrii Nakryiko , Xi Wang , Luke Nelson , KP Singh , Thomas Gleixner , linux-arm-kernel@lists.infradead.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, Greg Kroah-Hartman , bpf@vger.kernel.org, Martin KaFai Lau , Christoffer Dall Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.org Hi Luke, Thanks a lot for nailing these bugs. On Wed, 6 May 2020 18:05:01 -0700 Luke Nelson wrote: > This patch fixes two issues present in the current function for encoding > arm64 logical immediates when using the 32-bit variants of instructions. > > First, the code does not correctly reject an all-ones 32-bit immediate > and returns an undefined instruction encoding, which can crash the kernel. You make it sound more dramatic than it needs to be! ;-) As you pointed out below, nothing in the kernel calls this code to encode a 32bit immediate, so triggering a crash is not possible (unless you manage to exploit something else to call into this code). It definitely needs fixing though! > The fix is to add a check for this case. > > Second, the code incorrectly rejects some 32-bit immediates that are > actually encodable as logical immediates. The root cause is that the code > uses a default mask of 64-bit all-ones, even for 32-bit immediates. This > causes an issue later on when the mask is used to fill the top bits of > the immediate with ones, shown here: > > /* > * Pattern: 0..01..10..01..1 > * > * Fill the unused top bits with ones, and check if > * the result is a valid immediate (all ones with a > * contiguous ranges of zeroes). > */ > imm |= ~mask; > if (!range_of_ones(~imm)) > return AARCH64_BREAK_FAULT; > > To see the problem, consider an immediate of the form 0..01..10..01..1, > where the upper 32 bits are zero, such as 0x80000001. The code checks > if ~(imm | ~mask) contains a range of ones: the incorrect mask yields > 1..10..01..10..0, which fails the check; the correct mask yields > 0..01..10..0, which succeeds. > > The fix is to use a 32-bit all-ones default mask for 32-bit immediates. Paging this thing back in is really hard (I only had one coffee, more needed). Yes, I see what you mean. Duh! I think this only happens if mask hasn't been adjusted by the "pattern spotting" code the first place though. > > Currently, the only user of this function is in > arch/arm64/kvm/va_layout.c, which uses 64-bit immediates and won't > trigger these bugs. > > We tested the new code against llvm-mc with all 1,302 encodable 32-bit > logical immediates and all 5,334 encodable 64-bit logical immediates. That, on its own, is awesome information. Do you have any pointer on how to set this up? > > Fixes: ef3935eeebff ("arm64: insn: Add encoder for bitwise operations using literals") > Co-developed-by: Xi Wang > Signed-off-by: Xi Wang > Signed-off-by: Luke Nelson > --- > arch/arm64/kernel/insn.c | 6 ++++-- > 1 file changed, 4 insertions(+), 2 deletions(-) > > diff --git a/arch/arm64/kernel/insn.c b/arch/arm64/kernel/insn.c > index 4a9e773a177f..42fad79546bb 100644 > --- a/arch/arm64/kernel/insn.c > +++ b/arch/arm64/kernel/insn.c > @@ -1535,7 +1535,7 @@ static u32 aarch64_encode_immediate(u64 imm, > u32 insn) > { > unsigned int immr, imms, n, ones, ror, esz, tmp; > - u64 mask = ~0UL; > + u64 mask; > > /* Can't encode full zeroes or full ones */ > if (!imm || !~imm) > @@ -1543,13 +1543,15 @@ static u32 aarch64_encode_immediate(u64 imm, > > switch (variant) { > case AARCH64_INSN_VARIANT_32BIT: > - if (upper_32_bits(imm)) > + if (upper_32_bits(imm) || imm == 0xffffffffUL) nit: I don't like the fact that this create a small dissymmetry in the way we check things (we start by checking !~imm, which is not relevant to 32bit constants). > return AARCH64_BREAK_FAULT; > esz = 32; > + mask = 0xffffffffUL; > break; > case AARCH64_INSN_VARIANT_64BIT: > insn |= AARCH64_INSN_SF_BIT; > esz = 64; > + mask = ~0UL; I'd rather we generate the mask in a programmatic way, which is pretty easy to do since we have the initial element size. > break; > default: > pr_err("%s: unknown variant encoding %d\n", __func__, variant); To account for the above remarks, I came up with the following patch: diff --git a/arch/arm64/kernel/insn.c b/arch/arm64/kernel/insn.c index 4a9e773a177f..422bf9a79ed6 100644 --- a/arch/arm64/kernel/insn.c +++ b/arch/arm64/kernel/insn.c @@ -1535,11 +1535,7 @@ static u32 aarch64_encode_immediate(u64 imm, u32 insn) { unsigned int immr, imms, n, ones, ror, esz, tmp; - u64 mask = ~0UL; - - /* Can't encode full zeroes or full ones */ - if (!imm || !~imm) - return AARCH64_BREAK_FAULT; + u64 mask; switch (variant) { case AARCH64_INSN_VARIANT_32BIT: @@ -1556,6 +1552,11 @@ static u32 aarch64_encode_immediate(u64 imm, return AARCH64_BREAK_FAULT; } + /* Can't encode full zeroes or full ones */ + mask = GENMASK_ULL(esz - 1, 0); + if (!imm || !(~imm & mask)) + return AARCH64_BREAK_FAULT; + /* * Inverse of Replicate(). Try to spot a repeating pattern * with a pow2 stride. which is of course completely untested (it does compile though). Thoughts? M. -- Jazz is not dead. It just smells funny... _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel