[PATCH 6/8] elf: Allow arch to tweak initial mmap prot flags

[Dave Martin]

An arch may want to tweak the mmap prot flags for an ELF
executable's initial mappings.  For example, arm64 is going to need
to add PROT_BTI_GUARDED for executable pages in an ELF process
whose executable is marked as using Branch Target Identification
(an ARMv8.5-A control flow integrity feature).

So that this can be done in a generic way, add a hook
arch_elf_adjust_prot() to modify the prot flags as desired: arches
can select CONFIG_HAVE_ELF_PROT and implement their own backend
where necessary.

By default, leave the prot flags unchanged.

Signed-off-by: Dave Martin <Dave.Martin@xxxxxxx>
---
 fs/Kconfig.binfmt   |  3 +++
 fs/binfmt_elf.c     | 18 ++++++++++++------
 include/linux/elf.h | 13 +++++++++++++
 3 files changed, 28 insertions(+), 6 deletions(-)

diff --git a/fs/Kconfig.binfmt b/fs/Kconfig.binfmt
index 175a1f5..cd3d315 100644
--- a/fs/Kconfig.binfmt
+++ b/fs/Kconfig.binfmt
@@ -35,6 +35,9 @@ config COMPAT_BINFMT_ELF
 config ARCH_BINFMT_ELF_STATE
 	bool
 
+config ARCH_HAVE_ELF_PROT
+	bool
+
 config ARCH_USE_GNU_PROPERTY
 	bool
 	depends on 64BIT
diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index 32c9c13..3d88dcc 100644
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -524,7 +524,8 @@ static inline int arch_check_elf(struct elfhdr *ehdr, bool has_interp,
 
 #endif /* !CONFIG_ARCH_BINFMT_ELF_STATE */
 
-static inline int make_prot(u32 p_flags)
+static inline int make_prot(u32 p_flags, struct arch_elf_state *arch_state,
+			    bool has_interp, bool is_interp)
 {
 	int prot = 0;
 
@@ -534,7 +535,8 @@ static inline int make_prot(u32 p_flags)
 		prot |= PROT_WRITE;
 	if (p_flags & PF_X)
 		prot |= PROT_EXEC;
-	return prot;
+
+	return arch_elf_adjust_prot(prot, arch_state, has_interp, is_interp);
 }
 
 /* This is much more generalized than the library routine read function,
@@ -544,7 +546,8 @@ static inline int make_prot(u32 p_flags)
 
 static unsigned long load_elf_interp(struct elfhdr *interp_elf_ex,
 		struct file *interpreter, unsigned long *interp_map_addr,
-		unsigned long no_base, struct elf_phdr *interp_elf_phdata)
+		unsigned long no_base, struct elf_phdr *interp_elf_phdata,
+		struct arch_elf_state *arch_state)
 {
 	struct elf_phdr *eppnt;
 	unsigned long load_addr = 0;
@@ -576,7 +579,8 @@ static unsigned long load_elf_interp(struct elfhdr *interp_elf_ex,
 	for (i = 0; i < interp_elf_ex->e_phnum; i++, eppnt++) {
 		if (eppnt->p_type == PT_LOAD) {
 			int elf_type = MAP_PRIVATE | MAP_DENYWRITE;
-			int elf_prot = make_prot(eppnt->p_flags);
+			int elf_prot = make_prot(eppnt->p_flags, arch_state,
+						 true, true);
 			unsigned long vaddr = 0;
 			unsigned long k, map_addr;
 
@@ -952,7 +956,8 @@ static int load_elf_binary(struct linux_binprm *bprm)
 			elf_fixed = MAP_FIXED;
 		}
 
-		elf_prot = make_prot(elf_ppnt->p_flags);
+		elf_prot = make_prot(elf_ppnt->p_flags, &arch_state,
+				     !!interpreter, false);
 
 		elf_flags = MAP_PRIVATE | MAP_DENYWRITE | MAP_EXECUTABLE;
 
@@ -1098,7 +1103,8 @@ static int load_elf_binary(struct linux_binprm *bprm)
 		elf_entry = load_elf_interp(&loc->interp_elf_ex,
 					    interpreter,
 					    &interp_map_addr,
-					    load_bias, interp_elf_phdata);
+					    load_bias, interp_elf_phdata,
+					    &arch_state);
 		if (!IS_ERR((void *)elf_entry)) {
 			/*
 			 * load_elf_interp() returns relocation
diff --git a/include/linux/elf.h b/include/linux/elf.h
index cfcf154..2057187 100644
--- a/include/linux/elf.h
+++ b/include/linux/elf.h
@@ -75,4 +75,17 @@ static inline int arch_parse_property(void *ehdr, void *phdr, struct file *f,
 static inline int get_gnu_property(void *ehdr_p, void *phdr_p, struct file *f,
 				   u32 pr_type, u32 *feature) { return 0; }
 #endif
+
+#ifdef CONFIG_ARCH_HAVE_ELF_PROT
+int arch_elf_adjust_prot(int prot, const struct arch_elf_state *state,
+			 bool has_interp, bool is_interp);
+#else
+static inline int arch_elf_adjust_prot(int prot,
+				       const struct arch_elf_state *state,
+				       bool has_interp, bool is_interp)
+{
+	return prot;
+}
+#endif
+
 #endif /* _LINUX_ELF_H */
-- 
2.1.4