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[198.145.64.163]) by smtp.gmail.com with ESMTPSA id l18-20020a056a00141200b004f75395b2cesm856359pfu.150.2022.04.14.16.05.55 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 14 Apr 2022 16:05:55 -0700 (PDT) Date: Thu, 14 Apr 2022 16:05:54 -0700 From: Kees Cook To: Niklas Cassel Cc: Alexander Viro , Eric Biederman , Paul Walmsley , Palmer Dabbelt , Albert Ou , Greg Ungerer , Mike Frysinger , Damien Le Moal , stable@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-riscv@lists.infradead.org Subject: Re: [PATCH v2] binfmt_flat: do not stop relocating GOT entries prematurely on riscv Message-ID: <202204141559.B2A0EB4F7@keescook> References: <20220414091018.896737-1-niklas.cassel@wdc.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220414091018.896737-1-niklas.cassel@wdc.com> Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Thu, Apr 14, 2022 at 11:10:18AM +0200, Niklas Cassel wrote: > bFLT binaries are usually created using elf2flt. > > The linker script used by elf2flt has defined the .data section like the > following for the last 19 years: > > .data : { > _sdata = . ; > __data_start = . ; > data_start = . ; > *(.got.plt) > *(.got) > FILL(0) ; > . = ALIGN(0x20) ; > LONG(-1) > . = ALIGN(0x20) ; > ... > } > > It places the .got.plt input section before the .got input section. > The same is true for the default linker script (ld --verbose) on most > architectures except x86/x86-64. > > The binfmt_flat loader should relocate all GOT entries until it encounters > a -1 (the LONG(-1) in the linker script). > > The problem is that the .got.plt input section starts with a GOTPLT header > (which has size 16 bytes on elf64-riscv and 8 bytes on elf32-riscv), where > the first word is set to -1. See the binutils implementation for riscv [1]. > > This causes the binfmt_flat loader to stop relocating GOT entries > prematurely and thus causes the application to crash when running. > > Fix this by skipping the whole GOTPLT header, since the whole GOTPLT header > is reserved for the dynamic linker. > > The GOTPLT header will only be skipped for bFLT binaries with flag > FLAT_FLAG_GOTPIC set. This flag is unconditionally set by elf2flt if the > supplied ELF binary has the symbol _GLOBAL_OFFSET_TABLE_ defined. > ELF binaries without a .got input section should thus remain unaffected. > > Tested on RISC-V Canaan Kendryte K210 and RISC-V QEMU nommu_virt_defconfig. > > [1] https://sourceware.org/git/?p=binutils-gdb.git;a=blob;f=bfd/elfnn-riscv.c;hb=binutils-2_38#l3275 > > Cc: > Signed-off-by: Niklas Cassel > --- > Changes since v1: > -Incorporated review comments from Eric Biederman. Thanks! nit: please include a link to the archive so it's easier for people (and b4) to track earlier versions. i.e.: https://lore.kernel.org/linux-mm/20220412100338.437308-1-niklas.cassel@wdc.com/ > RISC-V elf2flt patches are still not merged, they can be found here: > https://github.com/floatious/elf2flt/tree/riscv > > buildroot branch for k210 nommu (including this patch and elf2flt patches): > https://github.com/floatious/buildroot/tree/k210-v14 > > fs/binfmt_flat.c | 27 ++++++++++++++++++++++++++- > 1 file changed, 26 insertions(+), 1 deletion(-) > > diff --git a/fs/binfmt_flat.c b/fs/binfmt_flat.c > index 626898150011..e5e2a03b39c1 100644 > --- a/fs/binfmt_flat.c > +++ b/fs/binfmt_flat.c > @@ -440,6 +440,30 @@ static void old_reloc(unsigned long rl) > > /****************************************************************************/ > > +static inline u32 __user *skip_got_header(u32 __user *rp) > +{ > + if (IS_ENABLED(CONFIG_RISCV)) { > + /* > + * RISC-V has a 16 byte GOT PLT header for elf64-riscv > + * and 8 byte GOT PLT header for elf32-riscv. > + * Skip the whole GOT PLT header, since it is reserved > + * for the dynamic linker (ld.so). > + */ > + u32 rp_val0, rp_val1; > + > + if (get_user(rp_val0, rp)) > + return rp; > + if (get_user(rp_val1, rp + 1)) > + return rp; > + > + if (rp_val0 == 0xffffffff && rp_val1 == 0xffffffff) > + rp += 4; > + else if (rp_val0 == 0xffffffff) > + rp += 2; Just so I understand; due to the FILL(0) and the ALIGN, val1 will be 0 (or more specifically, not -1) in all other cases, yes? I probably would have written this as: if (rp_val0 == 0xffffffff) rp += 2; if (rp_val1 == 0xffffffff) rp += 2; But no need to change it. I expect the compiler would optimize it in the same thing anyway. :) > + } > + return rp; > +} > + > static int load_flat_file(struct linux_binprm *bprm, > struct lib_info *libinfo, int id, unsigned long *extra_stack) > { > @@ -789,7 +813,8 @@ static int load_flat_file(struct linux_binprm *bprm, > * image. > */ > if (flags & FLAT_FLAG_GOTPIC) { > - for (rp = (u32 __user *)datapos; ; rp++) { > + rp = skip_got_header((u32 * __user) datapos); > + for (; ; rp++) { > u32 addr, rp_val; > if (get_user(rp_val, rp)) > return -EFAULT; > -- > 2.35.1 > Eric and Damien, does this look good to you? I'll take this into the execve tree unless you'd like to see further changes. Thanks! -Kees -- Kees Cook