[PATCH bpf-next v2 2/3] bpf: Add user memory access kfuncs for linux_binprm

[Anastasios Papagiannis <tasos.papagiannnis@gmail.com>]

When security_bprm_check runs, the arg and env strings for the exec have
been copied into bprm->mm. The new address space has not been associated
yet with a task_struct until exec_mmap(), so existing BPF user memory
helpers can only read from the calling task's old address space.

This patch adds bpf_copy_from_user_bprm() and
bpf_copy_from_user_bprm_str() kfuncs. Both use the mm_struct provided by
struct linux_binprm.

Register these kfuncs on both MMU and NOMMU systems. On NOMMU systems,
exec arguments are staged in bprm->page[] rather than mapped in bprm->mm,
so the kfuncs return -EOPNOTSUPP.

bpf_copy_from_user_bprm() has similar semantics as
bpf_copy_from_user_task(). bpf_copy_from_user_bprm_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.

This patch registers both kfuncs with KF_SLEEPABLE because accessing the
remote address space can fault. This allows BPF LSM programs attached to
security_bprm_check to read arguments beginning at bprm->p and reject an
exec based on its command-line arguments.

Signed-off-by: Anastasios Papagiannis <tasos.papagiannnis@gmail.com>
---
 fs/bpf_fs_kfuncs.c | 128 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 128 insertions(+)

diff --git a/fs/bpf_fs_kfuncs.c b/fs/bpf_fs_kfuncs.c
index f1863a891db6..1cd06f9e4890 100644
--- a/fs/bpf_fs_kfuncs.c
+++ b/fs/bpf_fs_kfuncs.c
@@ -1,6 +1,7 @@
 // SPDX-License-Identifier: GPL-2.0
 /* Copyright (c) 2024 Google LLC. */
 
+#include <linux/binfmts.h>
 #include <linux/bpf.h>
 #include <linux/bpf_lsm.h>
 #include <linux/btf.h>
@@ -379,6 +380,131 @@ __bpf_kfunc struct inode *bpf_real_data_inode(struct file *file)
 	return d_real_inode(file_dentry(file));
 }
 
+/**
+ * bpf_copy_from_user_bprm - Copy data from a binary parameter address space
+ * @dst:             Destination address, in kernel space
+ * @dst__sz:         Number of bytes to copy
+ * @unsafe_ptr__ign: Source address in the binary parameter address space
+ * @bprm:            Binary parameters whose address space will be used
+ * @flags:           Reserved for future use; must be zero
+ *
+ * Copies data from the nascent address space associated with @bprm. This is
+ * useful for reading the argument and environment strings before the new
+ * address space is installed by exec_mmap(). For example, at the
+ * bprm_check_security LSM hook, @bprm->p points at the first argument string.
+ *
+ * The destination is zeroed if the requested number of bytes cannot be copied
+ * in full.
+ *
+ * Return: 0 on success, -EINVAL if @flags is non-zero, -EOPNOTSUPP on
+ * NOMMU systems, or -EFAULT if the copy fails or is partial.
+ */
+__bpf_kfunc int bpf_copy_from_user_bprm(void *dst, u32 dst__sz,
+					const void __user *unsafe_ptr__ign,
+					const struct linux_binprm *bprm, u64 flags)
+{
+#ifdef CONFIG_MMU
+	struct mm_struct *mm;
+	int ret;
+#endif
+
+	if (unlikely(flags))
+		return -EINVAL;
+
+	if (unlikely(!dst__sz))
+		return 0;
+
+#ifdef CONFIG_MMU
+	mm = bprm->mm;
+	if (!mm) {
+		memset(dst, 0, dst__sz);
+		return -EFAULT;
+	}
+
+	ret = access_remote_vm(mm, (unsigned long)unsafe_ptr__ign,
+			       dst, dst__sz, 0);
+	if (ret != dst__sz) {
+		memset(dst, 0, dst__sz);
+		return -EFAULT;
+	}
+
+	return 0;
+#else
+	memset(dst, 0, dst__sz);
+	return -EOPNOTSUPP;
+#endif
+}
+
+/**
+ * bpf_copy_from_user_bprm_str - Copy a string from binary parameter memory
+ * @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 binary parameter address space
+ * @bprm:            Binary parameters whose address space will be used
+ * @flags:           The only supported flag is BPF_F_PAD_ZEROS
+ *
+ * Copies a NUL-terminated string from the nascent address space associated
+ * with @bprm. If the string is too long, @dst is still NUL-terminated unless
+ * @dst__sz is zero.
+ *
+ * If BPF_F_PAD_ZEROS is set, the unused portion of @dst is cleared on success
+ * and all of @dst is cleared on failure.
+ *
+ * Return: The number of copied bytes including the NUL terminator on success,
+ * or a negative error code on failure. On NOMMU systems, -EOPNOTSUPP is
+ * returned.
+ */
+__bpf_kfunc int bpf_copy_from_user_bprm_str(void *dst, u32 dst__sz,
+					    const void __user *unsafe_ptr__ign,
+					    const struct linux_binprm *bprm,
+					    u64 flags)
+{
+#ifdef CONFIG_MMU
+	struct mm_struct *mm;
+	int ret;
+#endif
+
+	if (unlikely(flags & ~BPF_F_PAD_ZEROS))
+		return -EINVAL;
+
+	if (unlikely(!dst__sz))
+		return 0;
+
+#ifdef CONFIG_MMU
+	mm = bprm->mm;
+	if (!mm) {
+		if (flags & BPF_F_PAD_ZEROS)
+			memset(dst, 0, dst__sz);
+		else
+			*(char *)dst = '\0';
+
+		return -EFAULT;
+	}
+
+	ret = copy_remote_mm_str(mm, (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;
+#else
+	if (flags & BPF_F_PAD_ZEROS)
+		memset(dst, 0, dst__sz);
+	else
+		*(char *)dst = '\0';
+
+	return -EOPNOTSUPP;
+#endif
+}
+
 __bpf_kfunc_end_defs();
 
 BTF_KFUNCS_START(bpf_fs_kfunc_set_ids)
@@ -390,6 +516,8 @@ BTF_ID_FLAGS(func, bpf_get_file_xattr, KF_SLEEPABLE)
 BTF_ID_FLAGS(func, bpf_set_dentry_xattr, KF_SLEEPABLE)
 BTF_ID_FLAGS(func, bpf_remove_dentry_xattr, KF_SLEEPABLE)
 BTF_ID_FLAGS(func, bpf_real_data_inode, KF_SLEEPABLE | KF_RET_NULL)
+BTF_ID_FLAGS(func, bpf_copy_from_user_bprm, KF_SLEEPABLE)
+BTF_ID_FLAGS(func, bpf_copy_from_user_bprm_str, KF_SLEEPABLE)
 BTF_KFUNCS_END(bpf_fs_kfunc_set_ids)
 
 static int bpf_fs_kfuncs_filter(const struct bpf_prog *prog, u32 kfunc_id)
-- 
2.55.0