[PATCH v3 06/13] loader: Explicitly munmap() the preloader's ELF EHDR.

[Jinoh Kang]

Today, the preloader reserves some predefined address ranges without
checking if there are any overlapping virtual memory mappings.

One side effect of this behaviour is that the preloader's ELF EHDR gets
unmapped.  Note the following overlapping address ranges:

- 0x00110000 - 0x68000000: low memory area (preload_info)
- 0x08040000 - 0x08041000: preloader ELF EHDR (x86)
- 0x00400000 - 0x00401000: preloader ELF EHDR (AMD64)

In practice, unmapping the preloader ELF EHDR is harmless; this is
because the dynamic linker does not recognise the preloader binary.

Make the unmapping behaviour explicit by calling munmap() on the
preloader's ELF EHDR.

Signed-off-by: Jinoh Kang <jinoh.kang.kr at gmail.com>
---

Notes:
    v1 -> v2: fix comparing text segment start against EHDR start

 loader/preloader.c | 30 ++++++++++++++++++++++++++++++
 1 file changed, 30 insertions(+)

diff --git a/loader/preloader.c b/loader/preloader.c
index fb22eff5f61..3f1d4ff7b21 100644
--- a/loader/preloader.c
+++ b/loader/preloader.c
@@ -223,6 +223,7 @@ struct
  *  then jumps to the address wld_start returns.
  */
 void _start(void);
+extern char __executable_start[];
 extern char _end[];
 __ASM_GLOBAL_FUNC(_start,
                   __ASM_CFI("\t.cfi_undefined %eip\n")
@@ -342,6 +343,15 @@ __ASM_GLOBAL_FUNC(wld_mmap,
                   __ASM_CFI(".cfi_adjust_cfa_offset -4\n\t")
                   "\tret\n" )
 
+static inline int wld_munmap( void *addr, size_t len )
+{
+    long ret;
+    __asm__ __volatile__( "int $0x80"
+                          : "=a" (ret) : "0" (91 /* SYS_munmap */), "b" (addr), "c" (len)
+                          : "memory" );
+    return SYSCALL_RET(ret);
+}
+
 static inline int wld_prctl( int code, long arg )
 {
     long ret;
@@ -361,6 +371,7 @@ void *thread_data[256];
  *  then jumps to the address wld_start returns.
  */
 void _start(void);
+extern char __executable_start[];
 extern char _end[];
 __ASM_GLOBAL_FUNC(_start,
                   __ASM_CFI(".cfi_undefined %rip\n\t")
@@ -424,6 +435,9 @@ SYSCALL_FUNC( wld_mmap, 9 /* SYS_mmap */ );
 int wld_mprotect( const void *addr, size_t len, int prot );
 SYSCALL_FUNC( wld_mprotect, 10 /* SYS_mprotect */ );
 
+int wld_munmap( void *addr, size_t len );
+SYSCALL_FUNC( wld_munmap, 11 /* SYS_munmap */ );
+
 int wld_prctl( int code, long arg );
 SYSCALL_FUNC( wld_prctl, 157 /* SYS_prctl */ );
 
@@ -450,6 +464,7 @@ void *thread_data[256];
  *  then jumps to the address wld_start returns.
  */
 void _start(void);
+extern char __executable_start[];
 extern char _end[];
 __ASM_GLOBAL_FUNC(_start,
                   "mov x0, SP\n\t"
@@ -530,6 +545,9 @@ SYSCALL_FUNC( wld_mmap, 222 /* SYS_mmap */ );
 int wld_mprotect( const void *addr, size_t len, int prot );
 SYSCALL_FUNC( wld_mprotect, 226 /* SYS_mprotect */ );
 
+int wld_munmap( void *addr, size_t len );
+SYSCALL_FUNC( wld_munmap, 215 /* SYS_munmap */ );
+
 int wld_prctl( int code, long arg );
 SYSCALL_FUNC( wld_prctl, 167 /* SYS_prctl */ );
 
@@ -556,6 +574,7 @@ void *thread_data[256];
  *  then jumps to the address wld_start returns.
  */
 void _start(void);
+extern char __executable_start[];
 extern char _end[];
 __ASM_GLOBAL_FUNC(_start,
                   "mov r0, sp\n\t"
@@ -628,6 +647,9 @@ void *wld_mmap( void *start, size_t len, int prot, int flags, int fd, off_t offs
 int wld_mprotect( const void *addr, size_t len, int prot );
 SYSCALL_FUNC( wld_mprotect, 125 /* SYS_mprotect */ );
 
+int wld_munmap( void *addr, size_t len );
+SYSCALL_FUNC( wld_munmap, 91 /* SYS_munmap */ );
+
 int wld_prctl( int code, long arg );
 SYSCALL_FUNC( wld_prctl, 172 /* SYS_prctl */ );
 
@@ -1527,6 +1549,14 @@ void* wld_start( void **stack )
     preloader_start = (char *)_start - ((unsigned long)_start & page_mask);
     preloader_end = (char *)((unsigned long)(_end + page_mask) & ~page_mask);
 
+    if ((unsigned long)preloader_start >= (unsigned long)__executable_start + page_size)
+    {
+        /* Unmap preloader's ELF EHDR */
+        wld_munmap( __executable_start,
+                    ((unsigned long)preloader_start -
+                     (unsigned long)__executable_start) & ~page_mask );
+    }
+
 #ifdef DUMP_AUX_INFO
     wld_printf( "stack = %p\n", state.s.stack );
     for( i = 0; i < state.s.argc; i++ ) wld_printf("argv[%lx] = %s\n", i, state.s.argv[i]);
-- 
2.31.1