[PATCH 1/2] fs: support $ORIGIN in ELF interpreter paths

[Farid Zakaria <farid.m.zakaria@gmail.com>]

Currently, standard ELF and ELF FDPIC loaders expect a fixed path to the
dynamic linker/interpreter (PT_INTERP). However, for systems utilizing
relocatable dynamic interpreters (such as Nix/store-based environments),
hardcoding this path is inflexible and breaks binary portability.

Introduce support for resolving the $ORIGIN placeholder in the ELF
interpreter path. This maps the dynamic linker relative to the path
of the binary being executed, matching user-space origin resolution.

To avoid code duplication, implement a shared 'resolve_elf_interpreter()'
helper in the VFS exec layer. For safety, limit detection strictly to
the prefix string "$ORIGIN" to prevent complex parsing exploits.

Assisted-by: Antigravity:Gemini-Pro
Signed-off-by: Farid Zakaria <farid.m.zakaria@gmail.com>
---
 fs/binfmt_elf.c         | 11 +++++++++--
 fs/binfmt_elf_fdpic.c   | 15 +++++++++++++--
 fs/exec.c               | 42 +++++++++++++++++++++++++++++++++++++++++
 include/linux/binfmts.h |  2 ++
 4 files changed, 66 insertions(+), 4 deletions(-)

diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index 16a56b6b3..af11f96ae 100644
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -872,7 +872,7 @@ static int load_elf_binary(struct linux_binprm *bprm)
 
 	elf_ppnt = elf_phdata;
 	for (i = 0; i < elf_ex->e_phnum; i++, elf_ppnt++) {
-		char *elf_interpreter;
+		char *elf_interpreter, *resolved_interp;
 
 		if (elf_ppnt->p_type == PT_GNU_PROPERTY) {
 			elf_property_phdata = elf_ppnt;
@@ -904,8 +904,15 @@ static int load_elf_binary(struct linux_binprm *bprm)
 		if (elf_interpreter[elf_ppnt->p_filesz - 1] != '\0')
 			goto out_free_interp;
 
-		interpreter = open_exec(elf_interpreter);
+		resolved_interp = resolve_elf_interpreter(bprm, elf_interpreter);
 		kfree(elf_interpreter);
+		if (IS_ERR(resolved_interp)) {
+			retval = PTR_ERR(resolved_interp);
+			goto out_free_ph;
+		}
+
+		interpreter = open_exec(resolved_interp);
+		kfree(resolved_interp);
 		retval = PTR_ERR(interpreter);
 		if (IS_ERR(interpreter))
 			goto out_free_ph;
diff --git a/fs/binfmt_elf_fdpic.c b/fs/binfmt_elf_fdpic.c
index 7e3108489..e85727d71 100644
--- a/fs/binfmt_elf_fdpic.c
+++ b/fs/binfmt_elf_fdpic.c
@@ -230,7 +230,9 @@ static int load_elf_fdpic_binary(struct linux_binprm *bprm)
 
 	for (i = 0; i < exec_params.hdr.e_phnum; i++, phdr++) {
 		switch (phdr->p_type) {
-		case PT_INTERP:
+		case PT_INTERP: {
+			char *resolved_interp;
+
 			retval = -ENOMEM;
 			if (phdr->p_filesz > PATH_MAX)
 				goto error;
@@ -259,7 +261,15 @@ static int load_elf_fdpic_binary(struct linux_binprm *bprm)
 			kdebug("Using ELF interpreter %s", interpreter_name);
 
 			/* replace the program with the interpreter */
-			interpreter = open_exec(interpreter_name);
+			resolved_interp = resolve_elf_interpreter(bprm, interpreter_name);
+			kfree(interpreter_name);
+			if (IS_ERR(resolved_interp)) {
+				retval = PTR_ERR(resolved_interp);
+				goto error;
+			}
+
+			interpreter = open_exec(resolved_interp);
+			kfree(resolved_interp);
 			retval = PTR_ERR(interpreter);
 			if (IS_ERR(interpreter)) {
 				interpreter = NULL;
@@ -284,6 +294,7 @@ static int load_elf_fdpic_binary(struct linux_binprm *bprm)
 
 			interp_params.hdr = *((struct elfhdr *) bprm->buf);
 			break;
+		}
 
 		case PT_LOAD:
 #ifdef CONFIG_MMU
diff --git a/fs/exec.c b/fs/exec.c
index b92fe7db1..0978ae613 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -2024,6 +2024,48 @@ static int __init init_fs_exec_sysctls(void)
 fs_initcall(init_fs_exec_sysctls);
 #endif /* CONFIG_SYSCTL */
 
+char *resolve_elf_interpreter(struct linux_binprm *bprm, const char *elf_interpreter)
+{
+	char *pathbuf, *path, *slash, *resolved;
+
+	if (strncmp(elf_interpreter, "$ORIGIN", 7) != 0) {
+		char *ret = kstrdup(elf_interpreter, GFP_KERNEL);
+
+		return ret ? ret : ERR_PTR(-ENOMEM);
+	}
+
+	pathbuf = kmalloc(PATH_MAX, GFP_KERNEL);
+	if (!pathbuf)
+		return ERR_PTR(-ENOMEM);
+
+	path = file_path(bprm->file, pathbuf, PATH_MAX);
+	if (IS_ERR(path)) {
+		kfree(pathbuf);
+		return (char *)path;
+	}
+
+	slash = strrchr(path, '/');
+	if (slash) {
+		if (slash == path)
+			*(slash + 1) = '\0';
+		else
+			*slash = '\0';
+	} else {
+		kfree(pathbuf);
+		char *ret = kstrdup(elf_interpreter, GFP_KERNEL);
+
+		return ret ? ret : ERR_PTR(-ENOMEM);
+	}
+
+	resolved = kasprintf(GFP_KERNEL, "%s%s", path, elf_interpreter + 7);
+	kfree(pathbuf);
+	if (!resolved)
+		return ERR_PTR(-ENOMEM);
+
+	return resolved;
+}
+EXPORT_SYMBOL(resolve_elf_interpreter);
+
 #ifdef CONFIG_EXEC_KUNIT_TEST
 #include "tests/exec_kunit.c"
 #endif
diff --git a/include/linux/binfmts.h b/include/linux/binfmts.h
index 2c77e383e..17419cd3d 100644
--- a/include/linux/binfmts.h
+++ b/include/linux/binfmts.h
@@ -150,4 +150,6 @@ extern ssize_t read_code(struct file *, unsigned long, loff_t, size_t);
 int kernel_execve(const char *filename,
 		  const char *const *argv, const char *const *envp);
 
+char *resolve_elf_interpreter(struct linux_binprm *bprm, const char *elf_interpreter);
+
 #endif /* _LINUX_BINFMTS_H */
-- 
2.51.2