binfmt_elf: allow arch code to examine PT_LOPROC ... PT_HIPROC headers

MIPS is introducing new variants of its O32 ABI which differ in their
handling of floating point, in order to enable a gradual transition
towards a world where mips32 binaries can take advantage of new hardware
features only available when configured for certain FP modes. In order
to do this ELF binaries are being augmented with a new section that
indicates, amongst other things, the FP mode requirements of the binary.
The presence & location of such a section is indicated by a program
header in the PT_LOPROC ... PT_HIPROC range.

In order to allow the MIPS architecture code to examine the program
header & section in question, pass all program headers in this range
to an architecture-specific arch_elf_pt_proc function. This function
may return an error if the header is deemed invalid or unsuitable for
the system, in which case that error will be returned from
load_elf_binary and upwards through the execve syscall.

A means is required for the architecture code to make a decision once
it is known that all such headers have been seen, but before it is too
late to return from an execve syscall. For this purpose the
arch_check_elf function is added, and called once, after all PT_LOPROC
to PT_HIPROC headers have been passed to arch_elf_pt_proc but before
the code which invoked execve has been lost. This enables the
architecture code to make a decision based upon all the headers present
in an ELF binary and its interpreter, as is required to forbid
conflicting FP ABI requirements between an ELF & its interpreter.

In order to allow data to be stored throughout the calls to the above
functions, struct arch_elf_state is introduced.

Finally a variant of the SET_PERSONALITY macro is introduced which
accepts a pointer to the struct arch_elf_state, allowing it to act
based upon state observed from the architecture specific program
headers.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: linux-fsdevel@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/7679/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

fs/Kconfig.binfmt

@@ -30,6 +30,9 @@ config COMPAT_BINFMT_ELF
 config ARCH_BINFMT_ELF_RANDOMIZE_PIE
 	bool
 
+config ARCH_BINFMT_ELF_STATE
+	bool
+
 config BINFMT_ELF_FDPIC
 	bool "Kernel support for FDPIC ELF binaries"
 	default y

fs/binfmt_elf.c

@@ -440,6 +440,74 @@ static struct elf_phdr *load_elf_phdrs(struct elfhdr *elf_ex,
 	return elf_phdata;
 }
 
+#ifndef CONFIG_ARCH_BINFMT_ELF_STATE
+
+/**
+ * struct arch_elf_state - arch-specific ELF loading state
+ *
+ * This structure is used to preserve architecture specific data during
+ * the loading of an ELF file, throughout the checking of architecture
+ * specific ELF headers & through to the point where the ELF load is
+ * known to be proceeding (ie. SET_PERSONALITY).
+ *
+ * This implementation is a dummy for architectures which require no
+ * specific state.
+ */
+struct arch_elf_state {
+};
+
+#define INIT_ARCH_ELF_STATE {}
+
+/**
+ * arch_elf_pt_proc() - check a PT_LOPROC..PT_HIPROC ELF program header
+ * @ehdr:	The main ELF header
+ * @phdr:	The program header to check
+ * @elf:	The open ELF file
+ * @is_interp:	True if the phdr is from the interpreter of the ELF being
+ *		loaded, else false.
+ * @state:	Architecture-specific state preserved throughout the process
+ *		of loading the ELF.
+ *
+ * Inspects the program header phdr to validate its correctness and/or
+ * suitability for the system. Called once per ELF program header in the
+ * range PT_LOPROC to PT_HIPROC, for both the ELF being loaded and its
+ * interpreter.
+ *
+ * Return: Zero to proceed with the ELF load, non-zero to fail the ELF load
+ *         with that return code.
+ */
+static inline int arch_elf_pt_proc(struct elfhdr *ehdr,
+				   struct elf_phdr *phdr,
+				   struct file *elf, bool is_interp,
+				   struct arch_elf_state *state)
+{
+	/* Dummy implementation, always proceed */
+	return 0;
+}
+
+/**
+ * arch_check_elf() - check a PT_LOPROC..PT_HIPROC ELF program header
+ * @ehdr:	The main ELF header
+ * @has_interp:	True if the ELF has an interpreter, else false.
+ * @state:	Architecture-specific state preserved throughout the process
+ *		of loading the ELF.
+ *
+ * Provides a final opportunity for architecture code to reject the loading
+ * of the ELF & cause an exec syscall to return an error. This is called after
+ * all program headers to be checked by arch_elf_pt_proc have been.
+ *
+ * Return: Zero to proceed with the ELF load, non-zero to fail the ELF load
+ *         with that return code.
+ */
+static inline int arch_check_elf(struct elfhdr *ehdr, bool has_interp,
+				 struct arch_elf_state *state)
+{
+	/* Dummy implementation, always proceed */
+	return 0;
+}
+
+#endif /* !CONFIG_ARCH_BINFMT_ELF_STATE */
+
 /* This is much more generalized than the library routine read function,
    so we keep this separate.  Technically the library read function
    is only provided so that we can read a.out libraries that have
@@ -611,6 +679,7 @@ static int load_elf_binary(struct linux_binprm *bprm)
 		struct elfhdr elf_ex;
 		struct elfhdr interp_elf_ex;
 	} *loc;
+	struct arch_elf_state arch_state = INIT_ARCH_ELF_STATE;
 
 	loc = kmalloc(sizeof(*loc), GFP_KERNEL);
 	if (!loc) {
@@ -705,12 +774,21 @@ static int load_elf_binary(struct linux_binprm *bprm)
 
 	elf_ppnt = elf_phdata;
 	for (i = 0; i < loc->elf_ex.e_phnum; i++, elf_ppnt++)
-		if (elf_ppnt->p_type == PT_GNU_STACK) {
+		switch (elf_ppnt->p_type) {
+		case PT_GNU_STACK:
 			if (elf_ppnt->p_flags & PF_X)
 				executable_stack = EXSTACK_ENABLE_X;
 			else
 				executable_stack = EXSTACK_DISABLE_X;
 			break;
+
+		case PT_LOPROC ... PT_HIPROC:
+			retval = arch_elf_pt_proc(&loc->elf_ex, elf_ppnt,
+						  bprm->file, false,
+						  &arch_state);
+			if (retval)
+				goto out_free_dentry;
+			break;
 		}
 
 	/* Some simple consistency checks for the interpreter */
@@ -728,16 +806,38 @@ static int load_elf_binary(struct linux_binprm *bprm)
 						   interpreter);
 		if (!interp_elf_phdata)
 			goto out_free_dentry;
+
+		/* Pass PT_LOPROC..PT_HIPROC headers to arch code */
+		elf_ppnt = interp_elf_phdata;
+		for (i = 0; i < loc->interp_elf_ex.e_phnum; i++, elf_ppnt++)
+			switch (elf_ppnt->p_type) {
+			case PT_LOPROC ... PT_HIPROC:
+				retval = arch_elf_pt_proc(&loc->interp_elf_ex,
+							  elf_ppnt, interpreter,
+							  true, &arch_state);
+				if (retval)
+					goto out_free_dentry;
+				break;
+			}
 	}
 
+	/*
+	 * Allow arch code to reject the ELF at this point, whilst it's
+	 * still possible to return an error to the code that invoked
+	 * the exec syscall.
+	 */
+	retval = arch_check_elf(&loc->elf_ex, !!interpreter, &arch_state);
+	if (retval)
+		goto out_free_dentry;
+
 	/* Flush all traces of the currently running executable */
 	retval = flush_old_exec(bprm);
 	if (retval)
 		goto out_free_dentry;
 
 	/* Do this immediately, since STACK_TOP as used in setup_arg_pages
 	   may depend on the personality.  */
-	SET_PERSONALITY(loc->elf_ex);
+	SET_PERSONALITY2(loc->elf_ex, &arch_state);
 	if (elf_read_implies_exec(loc->elf_ex, executable_stack))
 		current->personality |= READ_IMPLIES_EXEC;
 
@@ -929,6 +1029,7 @@ static int load_elf_binary(struct linux_binprm *bprm)
 		}
 	}
 
+	kfree(interp_elf_phdata);
 	kfree(elf_phdata);
 
 	set_binfmt(&elf_format);

include/linux/elf.h

@@ -15,6 +15,11 @@
 	set_personality(PER_LINUX | (current->personality & (~PER_MASK)))
 #endif
 
+#ifndef SET_PERSONALITY2
+#define SET_PERSONALITY2(ex, state) \
+	SET_PERSONALITY(ex)
+#endif
+
 #if ELF_CLASS == ELFCLASS32
 
 extern Elf32_Dyn _DYNAMIC [];