bpf: Add bpf_copy_from_user_task_str() kfunc

This new kfunc will be able to copy a zero-terminated C strings from
another task's address space. This is similar to `bpf_copy_from_user_str()`
but reads memory of specified task.

Signed-off-by: Jordan Rome <linux@jordanrome.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20250213152125.1837400-2-linux@jordanrome.com

1 files changed, 45 insertions, 0 deletions

diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index f27ce162427a..183298fc11ba 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -3067,6 +3067,50 @@ __bpf_kfunc int bpf_copy_from_user_str(void *dst, u32 dst__sz, const void __user
 	return ret + 1;
 }
 
+/**
+ * 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
+ *                   at least @dst__sz bytes long.
+ * @dst__sz:         Maximum number of bytes to copy, includes the trailing NUL.
+ * @unsafe_ptr__ign: Source address in the task's address space.
+ * @tsk:             The task whose address space will be used
+ * @flags:           The only supported flag is BPF_F_PAD_ZEROS
+ *
+ * Copies a NUL terminated string from a task's address space to @dst__sz
+ * buffer. If user string is too long this will still ensure zero termination
+ * in the @dst__sz buffer unless buffer size is 0.
+ *
+ * If BPF_F_PAD_ZEROS flag is set, memset the tail of @dst__sz to 0 on success
+ * and memset all of @dst__sz on failure.
+ *
+ * Return: The number of copied bytes on success including the NUL terminator.
+ * A negative error code on failure.
+ */
+__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)
+{
+	int ret;
+
+	if (unlikely(flags & ~BPF_F_PAD_ZEROS))
+		return -EINVAL;
+
+	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) {
+		if (flags & BPF_F_PAD_ZEROS)
+			memset(dst, 0, dst__sz);
+		return ret;
+	}
+
+	if (flags & BPF_F_PAD_ZEROS)
+		memset(dst + ret, 0, dst__sz - ret);
+
+	return ret + 1;
+}
+
 /* Keep unsinged long in prototype so that kfunc is usable when emitted to
  * vmlinux.h in BPF programs directly, but note that while in BPF prog, the
  * unsigned long always points to 8-byte region on stack, the kernel may only
@@ -3174,6 +3218,7 @@ 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_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)
 BTF_ID_FLAGS(func, bpf_iter_kmem_cache_next, KF_ITER_NEXT | KF_RET_NULL | KF_SLEEPABLE)