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Thu, 17 Sep 2026 18:05:05 -0700 (PDT) Date: Thu, 17 Sep 2026 21:04:37 -0400 From: Matt Bobrowski To: Anastasios Papagiannis Cc: bpf@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, david@kernel.org, akpm@linux-foundation.org, andrii@kernel.org, ast@kernel.org, brauner@kernel.org, daniel@iogearbox.net, eddyz87@gmail.com, kpsingh@kernel.org, ljs@kernel.org, memxor@gmail.com, song@kernel.org, sun.jian.kdev@gmail.com, utilityemal77@gmail.com, viro@zeniv.linux.org.uk Subject: Re: [PATCH bpf-next v7 3/5] bpf: Add user memory access kfuncs for mm_struct Message-ID: References: <20260915080255.48929-1-tasos.papagiannnis@gmail.com> <20260915080255.48929-4-tasos.papagiannnis@gmail.com> Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260915080255.48929-4-tasos.papagiannnis@gmail.com> On Tue, Sep 15, 2026 at 11:02:53AM +0300, Anastasios Papagiannis wrote: > On CONFIG_MMU kernels, when security_bprm_check() runs, the argument and > environment strings for the exec have been copied into bprm->mm. The new > address space is not associated with a task_struct until exec_mmap(), so > existing BPF user memory helpers cannot access it. > > Add bpf_copy_from_user_mm() and bpf_copy_from_user_mm_str() kfuncs. Both > take a struct mm_struct pointer directly, allowing callers to access > trusted address spaces that are not associated with a task_struct. > > bpf_copy_from_user_mm() has similar semantics to > bpf_copy_from_user_task(). bpf_copy_from_user_mm_str() copies one > NUL-terminated string and returns its size including the NUL terminator. > It accepts BPF_F_PAD_ZEROS to clear unused destination bytes on success. > > Refactor the task-based helpers and the new mm-based kfuncs to share > static internal implementations. The task-based interfaces validate their > arguments before acquiring and holding a reference to the task's mm for > the copy. No behavior change is intended for the existing task-based > interfaces. > > Register both new kfuncs and mark them KF_SLEEPABLE because accessing a > remote address space can fault. Thank you for employing my suggestions. I think this looks much better to me now. Feel free to add: Reviewed-by: Matt Bobrowski > Signed-off-by: Anastasios Papagiannis > --- > kernel/bpf/helpers.c | 130 +++++++++++++++++++++++++++++++++++++++---- > 1 file changed, 118 insertions(+), 12 deletions(-) > > diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c > index 051b6654e57c..f6b3eee6098a 100644 > --- a/kernel/bpf/helpers.c > +++ b/kernel/bpf/helpers.c > @@ -679,9 +679,44 @@ const struct bpf_func_proto bpf_copy_from_user_proto = { > .arg3_type = ARG_ANYTHING, > }; > > +static int __bpf_copy_from_user_mm(void *dst, u32 size, > + const void __user *user_ptr, > + struct mm_struct *mm) > +{ > + int ret; > + > + ret = access_remote_vm(mm, (unsigned long)user_ptr, dst, size, 0); > + if (ret == size) > + return 0; > + > + memset(dst, 0, size); > + /* Return -EFAULT for partial read */ > + return ret < 0 ? ret : -EFAULT; > +} > + > +static int __bpf_copy_from_user_mm_str(void *dst, u32 size, > + const void __user *user_ptr, > + struct mm_struct *mm, u64 flags) > +{ > + int ret; > + > + ret = copy_remote_mm_str(mm, (unsigned long)user_ptr, dst, size, 0); > + if (ret < 0) { > + if (flags & BPF_F_PAD_ZEROS) > + memset(dst, 0, size); > + return ret; > + } > + > + if (flags & BPF_F_PAD_ZEROS) > + memset(dst + ret, 0, size - ret); > + > + return ret + 1; > +} > + > BPF_CALL_5(bpf_copy_from_user_task, void *, dst, u32, size, > const void __user *, user_ptr, struct task_struct *, tsk, u64, flags) > { > + struct mm_struct *mm; > int ret; > > /* flags is not used yet */ > @@ -691,13 +726,16 @@ BPF_CALL_5(bpf_copy_from_user_task, void *, dst, u32, size, > if (unlikely(!size)) > return 0; > > - ret = access_process_vm(tsk, (unsigned long)user_ptr, dst, size, 0); > - if (ret == size) > - return 0; > + mm = get_task_mm(tsk); > + if (!mm) { > + memset(dst, 0, size); > + return -EFAULT; > + } > > - memset(dst, 0, size); > - /* Return -EFAULT for partial read */ > - return ret < 0 ? ret : -EFAULT; > + ret = __bpf_copy_from_user_mm(dst, size, user_ptr, mm); > + mmput(mm); > + > + return ret; > } > > const struct bpf_func_proto bpf_copy_from_user_task_proto = { > @@ -3659,6 +3697,68 @@ __bpf_kfunc int bpf_copy_from_user_str(void *dst, u32 dst__sz, const void __user > return ret + 1; > } > > +/** > + * bpf_copy_from_user_mm() - Copy data from an address space > + * @dst: Destination address, in kernel space > + * @dst__sz: Number of bytes to copy > + * @unsafe_ptr__ign: Source address in the address space > + * @mm: Address space to copy from > + * @flags: Reserved for future use; must be zero > + * > + * Copies data from the user address space associated with @mm. The destination > + * is zeroed if an attempted copy cannot be completed in full. Unsupported > + * flags return -EINVAL without modifying @dst. > + * > + * Return: 0 on success, -EINVAL if @flags is non-zero, or -EFAULT if the copy > + * fails or is partial. > + */ > +__bpf_kfunc int bpf_copy_from_user_mm(void *dst, u32 dst__sz, > + const void __user *unsafe_ptr__ign, > + struct mm_struct *mm, u64 flags) > +{ > + if (unlikely(flags)) > + return -EINVAL; > + > + if (unlikely(!dst__sz)) > + return 0; > + > + return __bpf_copy_from_user_mm(dst, dst__sz, unsafe_ptr__ign, mm); > +} > + > +/** > + * bpf_copy_from_user_mm_str() - Copy a string from an address space > + * @dst: Destination address, in kernel space. This buffer must be > + * at least @dst__sz bytes long > + * @dst__sz: Maximum number of bytes to copy, including the trailing NUL > + * @unsafe_ptr__ign: Source address in the address space > + * @mm: Address space to copy from > + * @flags: The only supported flag is BPF_F_PAD_ZEROS > + * > + * Copies a NUL-terminated string from the user address space associated with > + * @mm. If the string is too long, @dst is still NUL-terminated unless @dst__sz > + * is zero. > + * > + * If the flags are valid and BPF_F_PAD_ZEROS is set, the unused portion of > + * @dst is cleared on success and all of @dst is cleared on a copy failure. > + * Unsupported flags return -EINVAL without modifying @dst. > + * > + * Return: The number of copied bytes including the NUL terminator on success, > + * or a negative error code on failure. > + */ > +__bpf_kfunc int bpf_copy_from_user_mm_str(void *dst, u32 dst__sz, > + const void __user *unsafe_ptr__ign, > + struct mm_struct *mm, u64 flags) > +{ > + if (unlikely(flags & ~BPF_F_PAD_ZEROS)) > + return -EINVAL; > + > + if (unlikely(dst__sz == 0)) > + return 0; > + > + return __bpf_copy_from_user_mm_str(dst, dst__sz, unsafe_ptr__ign, > + mm, flags); > +} > + > /** > * bpf_copy_from_user_task_str() - Copy a string from an task's address space > * @dst: Destination address, in kernel space. This buffer must be > @@ -3682,6 +3782,7 @@ __bpf_kfunc int bpf_copy_from_user_task_str(void *dst, u32 dst__sz, > const void __user *unsafe_ptr__ign, > struct task_struct *tsk, u64 flags) > { > + struct mm_struct *mm; > int ret; > > if (unlikely(flags & ~BPF_F_PAD_ZEROS)) > @@ -3690,17 +3791,20 @@ __bpf_kfunc int bpf_copy_from_user_task_str(void *dst, u32 dst__sz, > if (unlikely(dst__sz == 0)) > return 0; > > - ret = copy_remote_vm_str(tsk, (unsigned long)unsafe_ptr__ign, dst, dst__sz, 0); > - if (ret < 0) { > + mm = get_task_mm(tsk); > + if (!mm) { > if (flags & BPF_F_PAD_ZEROS) > memset(dst, 0, dst__sz); > - return ret; > + else > + *(char *)dst = '\0'; > + return -EFAULT; > } > > - if (flags & BPF_F_PAD_ZEROS) > - memset(dst + ret, 0, dst__sz - ret); > + ret = __bpf_copy_from_user_mm_str(dst, dst__sz, unsafe_ptr__ign, > + mm, flags); > + mmput(mm); > > - return ret + 1; > + return ret; > } > > /* Keep unsigned long in prototype so that kfunc is usable when emitted to > @@ -4925,6 +5029,8 @@ BTF_ID_FLAGS(func, bpf_iter_bits_new, KF_ITER_NEW) > BTF_ID_FLAGS(func, bpf_iter_bits_next, KF_ITER_NEXT | KF_RET_NULL) > BTF_ID_FLAGS(func, bpf_iter_bits_destroy, KF_ITER_DESTROY) > BTF_ID_FLAGS(func, bpf_copy_from_user_str, KF_SLEEPABLE) > +BTF_ID_FLAGS(func, bpf_copy_from_user_mm, KF_SLEEPABLE) > +BTF_ID_FLAGS(func, bpf_copy_from_user_mm_str, KF_SLEEPABLE) > BTF_ID_FLAGS(func, bpf_copy_from_user_task_str, KF_SLEEPABLE) > BTF_ID_FLAGS(func, bpf_get_kmem_cache) > BTF_ID_FLAGS(func, bpf_iter_kmem_cache_new, KF_ITER_NEW | KF_SLEEPABLE) > -- > 2.55.0 >