[PATCH 3/4] aarch64: Use mmap to add PROT_BTI instead of mprotect [BZ #26831]

[Szabolcs Nagy <szabolcs.nagy@arm.com>]

Re-mmap executable segments if possible instead of using mprotect
to add PROT_BTI. This allows using BTI protection with security
policies that prevent mprotect with PROT_EXEC.

If the fd of the ELF module is not available because it was kernel
mapped then mprotect is used and failures are ignored.  It is
expected that linux kernel will add PROT_BTI when mapping a module
(current linux as of version 5.9 does not do this).

Computing the mapping parameters follows the logic of
_dl_map_object_from_fd more closely now.

Fixes bug 26831.
---
 sysdeps/aarch64/dl-bti.c  | 46 ++++++++++++++++++++-------------------
 sysdeps/aarch64/dl-prop.h | 14 +++++++-----
 2 files changed, 33 insertions(+), 27 deletions(-)

diff --git a/sysdeps/aarch64/dl-bti.c b/sysdeps/aarch64/dl-bti.c
index 196e462520..385f1731ca 100644
--- a/sysdeps/aarch64/dl-bti.c
+++ b/sysdeps/aarch64/dl-bti.c
@@ -19,43 +19,45 @@
 #include <errno.h>
 #include <libintl.h>
 #include <ldsodefs.h>
+#include <dl-load.h>  /* For MAP_COPY.  */
 
-static int
-enable_bti (struct link_map *map, const char *program)
+/* Enable BTI protection for MAP.  */
+
+void
+_dl_bti_protect (struct link_map *map, int fd)
 {
+  const size_t pagesz = GLRO(dl_pagesize);
   const ElfW(Phdr) *phdr;
-  unsigned prot;
 
   for (phdr = map->l_phdr; phdr < &map->l_phdr[map->l_phnum]; ++phdr)
     if (phdr->p_type == PT_LOAD && (phdr->p_flags & PF_X))
       {
-	void *start = (void *) (phdr->p_vaddr + map->l_addr);
-	size_t len = phdr->p_memsz;
-
-	prot = PROT_EXEC | PROT_BTI;
+	size_t vstart = ALIGN_DOWN (phdr->p_vaddr, pagesz);
+	size_t vend = ALIGN_UP (phdr->p_vaddr + phdr->p_filesz, pagesz);
+	off_t off = ALIGN_DOWN (phdr->p_offset, pagesz);
+	void *start = (void *) (vstart + map->l_addr);
+	size_t len = vend - vstart;
+
+	/* Add PROT_BTI.  */
+	unsigned prot = PROT_EXEC | PROT_BTI;
 	if (phdr->p_flags & PF_R)
 	  prot |= PROT_READ;
 	if (phdr->p_flags & PF_W)
 	  prot |= PROT_WRITE;
 
-	if (__mprotect (start, len, prot) < 0)
+	if (fd == -1)
+	  {
+	    /* Ignore failures: rely on the kernel adding PROT_BTI then.  */
+	    __mprotect (start, len, prot);
+	  }
+	else
 	  {
-	    if (program)
-	      _dl_fatal_printf ("%s: mprotect failed to turn on BTI\n",
-				map->l_name);
-	    else
+	    void *p = __mmap (start, len, prot, MAP_FIXED|MAP_COPY|MAP_FILE,
+			      fd, off);
+	    if (p == MAP_FAILED)
 	      _dl_signal_error (errno, map->l_name, "dlopen",
-				N_("mprotect failed to turn on BTI"));
+				N_("failed to turn on BTI protection"));
 	  }
       }
   return 0;
 }
-
-/* Enable BTI for L if required.  */
-
-void
-_dl_bti_check (struct link_map *l, const char *program)
-{
-  if (GLRO(dl_aarch64_cpu_features).bti && l->l_mach.bti)
-    enable_bti (l, program);
-}
diff --git a/sysdeps/aarch64/dl-prop.h b/sysdeps/aarch64/dl-prop.h
index 2016d1472e..762bc93733 100644
--- a/sysdeps/aarch64/dl-prop.h
+++ b/sysdeps/aarch64/dl-prop.h
@@ -19,19 +19,16 @@
 #ifndef _DL_PROP_H
 #define _DL_PROP_H
 
-extern void _dl_bti_check (struct link_map *, const char *)
-    attribute_hidden;
+extern void _dl_bti_protect (struct link_map *, int) attribute_hidden;
 
 static inline void __attribute__ ((always_inline))
 _rtld_main_check (struct link_map *m, const char *program)
 {
-  _dl_bti_check (m, program);
 }
 
 static inline void __attribute__ ((always_inline))
 _dl_open_check (struct link_map *m)
 {
-  _dl_bti_check (m, NULL);
 }
 
 static inline void __attribute__ ((always_inline))
@@ -43,6 +40,10 @@ static inline int
 _dl_process_gnu_property (struct link_map *l, int fd, uint32_t type,
 			  uint32_t datasz, void *data)
 {
+  if (!GLRO(dl_aarch64_cpu_features).bti)
+    /* Skip note processing.  */
+    return 0;
+
   if (type == GNU_PROPERTY_AARCH64_FEATURE_1_AND)
     {
       /* Stop if the property note is ill-formed.  */
@@ -51,7 +52,10 @@ _dl_process_gnu_property (struct link_map *l, int fd, uint32_t type,
 
       unsigned int feature_1 = *(unsigned int *) data;
       if (feature_1 & GNU_PROPERTY_AARCH64_FEATURE_1_BTI)
-	l->l_mach.bti = true;
+	{
+	  l->l_mach.bti = true;  /* No longer needed.  */
+	  _dl_bti_protect (l, fd);
+	}
 
       /* Stop if we processed the property note.  */
       return 0;
-- 
2.17.1