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To: Kumar Kartikeya Dwivedi Cc: bpf , Daniel Borkmann , Andrii Nakryiko , Eddy Z , Tejun Heo , Barret Rhoden , Johannes Weiner , Lorenzo Stoakes , Andrew Morton , Uladzislau Rezki , Christoph Hellwig , linux-mm , Kernel Team Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspamd-Queue-Id: 7FA6A100014 X-Rspam-User: X-Rspamd-Server: rspam02 X-Stat-Signature: abi5yjehnpsc655yx71zpxerhhqkrgap X-HE-Tag: 1707863342-899233 X-HE-Meta: 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 GOtwCqV8 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Sat, Feb 10, 2024 at 12:40=E2=80=AFAM Kumar Kartikeya Dwivedi wrote: > > On Fri, 9 Feb 2024 at 05:06, Alexei Starovoitov > wrote: > > > > From: Alexei Starovoitov > > > > LLVM generates bpf_cast_kern and bpf_cast_user instructions while trans= lating > > pointers with __attribute__((address_space(1))). > > > > rX =3D cast_kern(rY) is processed by the verifier and converted to > > normal 32-bit move: wX =3D wY > > > > bpf_cast_user has to be converted by JIT. > > > > rX =3D cast_user(rY) is > > > > aux_reg =3D upper_32_bits of arena->user_vm_start > > aux_reg <<=3D 32 > > wX =3D wY // clear upper 32 bits of dst register > > if (wX) // if not zero add upper bits of user_vm_start > > wX |=3D aux_reg > > > > Would this be ok if the rY is somehow aligned at the 4GB boundary, and > the lower 32-bits end up being zero. > Then this transformation would confuse it with the NULL case, right? yes. it will. I tried to fix it by reserving a zero page, but the end result was bad. See discussion with Barret. So we decided to drop this idea. Might come back to it eventually. Also, I was thinking of doing if (rX) instead of if (wX) to mitigate a bit, but that is probably wrong too. The best is to mitigate this inside bpf program by never returning lo32 zer= o from bpf_alloc() function. In general with the latest llvm we see close to zero cast_user when bpf prog is not mixing (void *) with (void __arena *) casts, so it shouldn't be an issue in practice with patches as-is.