elf: Validate elf section header table index for section name string table

In multiboot_elfxx.c, e_shstrndx is used to obtain the section header table
index of the section name string table, but it wasn't being checked if the value
was there.

According to the elf(5) manual page,
"If the index of section name string table section is larger than or equal to
SHN_LORESERVE (0xff00), this member holds SHN_XINDEX (0xffff) and the real
index of the section name string table section is held in the sh_link member of
the initial entry in section header table. Otherwise, the sh_link member of the
initial entry in section header table contains the value zero."

Since this check wasn't being made, grub_elfXX_get_shstrndx() is being added to
elfXX.c to make this check and use e_shstrndx if it doesn't have SHN_XINDEX as a
value, else use sh_link. We also need to make sure e_shstrndx isn't greater than
or equal to SHN_LORESERVE and sh_link isn't less than SHN_LORESERVE.

Note that even though elf.c and elfXX.c are located in grub-core/kern, they are
compiled as modules and don't need the EXPORT_FUNC() macro to define the functions
in elf.h.

Signed-off-by: Alec Brown <alec.r.brown@oracle.com>
Reviewed-by: Darren Kenny <darren.kenny@oracle.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

4 files changed, 52 insertions, 1 deletions

diff --git a/grub-core/kern/elf.c b/grub-core/kern/elf.c
index 66b69293e..c676db694 100644
--- a/grub-core/kern/elf.c
+++ b/grub-core/kern/elf.c
@@ -176,6 +176,7 @@ grub_elf_open (const char *name, enum grub_file_type type)
 #define grub_swap_bytes_XwordXX grub_swap_bytes32
 #define grub_elfXX_check_endianess_and_bswap_ehdr grub_elf32_check_endianess_and_bswap_ehdr
 #define grub_elfXX_get_shnum grub_elf32_get_shnum
+#define grub_elfXX_get_shstrndx grub_elf32_get_shstrndx
 
 #include "elfXX.c"
 
@@ -198,6 +199,7 @@ grub_elf_open (const char *name, enum grub_file_type type)
 #undef grub_swap_bytes_XwordXX
 #undef grub_elfXX_check_endianess_and_bswap_ehdr
 #undef grub_elfXX_get_shnum
+#undef grub_elfXX_get_shstrndx
 
 
 /* 64-bit */
@@ -220,5 +222,6 @@ grub_elf_open (const char *name, enum grub_file_type type)
 #define grub_swap_bytes_XwordXX grub_swap_bytes64
 #define grub_elfXX_check_endianess_and_bswap_ehdr grub_elf64_check_endianess_and_bswap_ehdr
 #define grub_elfXX_get_shnum grub_elf64_get_shnum
+#define grub_elfXX_get_shstrndx grub_elf64_get_shstrndx
 
 #include "elfXX.c"
diff --git a/grub-core/kern/elfXX.c b/grub-core/kern/elfXX.c
index 48b7745a5..4e3254fa5 100644
--- a/grub-core/kern/elfXX.c
+++ b/grub-core/kern/elfXX.c
@@ -239,3 +239,36 @@ grub_elfXX_get_shnum (ElfXX_Ehdr *e, ElfXX_Shnum *shnum)
 
   return GRUB_ERR_NONE;
 }
+
+grub_err_t
+grub_elfXX_get_shstrndx (ElfXX_Ehdr *e, ElfXX_Word *shstrndx)
+{
+  ElfXX_Shdr *s;
+
+  if (shstrndx == NULL)
+    return grub_error (GRUB_ERR_BAD_ARGUMENT, N_("NULL pointer passed for shstrndx"));
+
+  /* Set *shstrndx to 0 so that shstrndx doesn't return junk on error */
+  *shstrndx = 0;
+
+  if (e == NULL)
+    return grub_error (GRUB_ERR_BAD_ARGUMENT, N_("NULL pointer passed for elf header"));
+
+  *shstrndx = e->e_shstrndx;
+  if (*shstrndx == SHN_XINDEX)
+    {
+      if (e->e_shoff == 0)
+	return grub_error (GRUB_ERR_BAD_NUMBER, N_("invalid section header table offset in e_shoff"));
+
+      s = (ElfXX_Shdr *) ((grub_uint8_t *) e + e->e_shoff);
+      *shstrndx = s->sh_link;
+      if (*shstrndx < SHN_LORESERVE)
+	return grub_error (GRUB_ERR_BAD_NUMBER, N_("invalid section header table index in sh_link: %d"), *shstrndx);
+    }
+  else
+    {
+      if (*shstrndx >= SHN_LORESERVE)
+	return grub_error (GRUB_ERR_BAD_NUMBER, N_("invalid section header table index in e_shstrndx: %d"), *shstrndx);
+    }
+  return GRUB_ERR_NONE;
+}
diff --git a/grub-core/loader/multiboot_elfxx.c b/grub-core/loader/multiboot_elfxx.c
index ed5a4ae9d..ec4ba3b8d 100644
--- a/grub-core/loader/multiboot_elfxx.c
+++ b/grub-core/loader/multiboot_elfxx.c
@@ -23,8 +23,10 @@
 # define Elf_Ehdr		Elf32_Ehdr
 # define Elf_Phdr		Elf32_Phdr
 # define Elf_Shdr		Elf32_Shdr
+# define Elf_Word		Elf32_Word
 # define Elf_Shnum		Elf32_Shnum
 # define grub_multiboot_elf_get_shnum		grub_elf32_get_shnum
+# define grub_multiboot_elf_get_shstrndx	grub_elf32_get_shstrndx
 #elif defined(MULTIBOOT_LOAD_ELF64)
 # define XX			64
 # define E_MACHINE		MULTIBOOT_ELF64_MACHINE
@@ -32,8 +34,10 @@
 # define Elf_Ehdr		Elf64_Ehdr
 # define Elf_Phdr		Elf64_Phdr
 # define Elf_Shdr		Elf64_Shdr
+# define Elf_Word		Elf64_Word
 # define Elf_Shnum		Elf64_Shnum
 # define grub_multiboot_elf_get_shnum		grub_elf64_get_shnum
+# define grub_multiboot_elf_get_shstrndx	grub_elf64_get_shstrndx
 #else
 #error "I'm confused"
 #endif
@@ -63,6 +67,7 @@ CONCAT(grub_multiboot_load_elf, XX) (mbi_load_data_t *mld)
   grub_relocator_chunk_t ch;
   grub_uint32_t load_offset = 0, load_size;
   Elf_Shnum shnum;
+  Elf_Word shstrndx;
   unsigned int i;
   void *source = NULL;
 
@@ -84,6 +89,10 @@ CONCAT(grub_multiboot_load_elf, XX) (mbi_load_data_t *mld)
   if (err != GRUB_ERR_NONE)
     return err;
 
+  err = grub_multiboot_elf_get_shstrndx (ehdr, &shstrndx);
+  if (err != GRUB_ERR_NONE)
+    return err;
+
   /* FIXME: Should we support program headers at strange locations?  */
   if (ehdr->e_phoff + (grub_uint32_t) ehdr->e_phnum * ehdr->e_phentsize > MULTIBOOT_SEARCH)
     return grub_error (GRUB_ERR_BAD_OS, "program header at a too high offset");
@@ -291,7 +300,7 @@ CONCAT(grub_multiboot_load_elf, XX) (mbi_load_data_t *mld)
 	  sh->sh_addr = target;
 	}
       GRUB_MULTIBOOT (add_elfsyms) (shnum, ehdr->e_shentsize,
-				    ehdr->e_shstrndx, shdr);
+				    shstrndx, shdr);
     }
 
 #undef phdr
@@ -305,5 +314,7 @@ CONCAT(grub_multiboot_load_elf, XX) (mbi_load_data_t *mld)
 #undef Elf_Ehdr
 #undef Elf_Phdr
 #undef Elf_Shdr
+#undef Elf_Word
 #undef Elf_Shnum
 #undef grub_multiboot_elf_get_shnum
+#undef grub_multiboot_elf_get_shstrndx
diff --git a/include/grub/elf.h b/include/grub/elf.h
index ce2f5609a..43c4809ed 100644
--- a/include/grub/elf.h
+++ b/include/grub/elf.h
@@ -2536,8 +2536,10 @@ typedef Elf32_Addr Elf32_Conflict;
 #define R_RISCV_32_PCREL        57
 
 extern grub_err_t grub_elf32_get_shnum (Elf32_Ehdr *e, Elf32_Shnum *shnum);
+extern grub_err_t grub_elf32_get_shstrndx (Elf32_Ehdr *e, Elf32_Word *shstrndx);
 
 extern grub_err_t grub_elf64_get_shnum (Elf64_Ehdr *e, Elf64_Shnum *shnum);
+extern grub_err_t grub_elf64_get_shstrndx (Elf64_Ehdr *e, Elf64_Word *shstrndx);
 
 #ifdef GRUB_TARGET_WORDSIZE
 #if GRUB_TARGET_WORDSIZE == 32
@@ -2567,6 +2569,7 @@ typedef Elf32_Shnum Elf_Shnum;
 #define ELF_R_INFO(sym, type)	ELF32_R_INFO(sym, type)
 
 #define grub_elf_get_shnum	grub_elf32_get_shnum
+#define grub_elf_get_shstrndx	grub_elf32_get_shstrndx
 
 #elif GRUB_TARGET_WORDSIZE == 64
 
@@ -2594,6 +2597,7 @@ typedef Elf64_Shnum Elf_Shnum;
 #define ELF_R_INFO(sym, type)	ELF64_R_INFO(sym, type)
 
 #define grub_elf_get_shnum	grub_elf64_get_shnum
+#define grub_elf_get_shstrndx	grub_elf64_get_shstrndx
 
 #endif /* GRUB_TARGET_WORDSIZE == 64 */
 #endif