[PATCH v3 04/16] module: Factor out elf_validity_cache_secstrings

[Matthew Maurer <mmaurer@google.com>]

Factor out the validation of section names.

There are two behavioral changes:

1. Previously, we did not validate non-SHF_ALLOC sections.
   This may have once been safe, as find_sec skips non-SHF_ALLOC
   sections, but find_any_sec, which will be used to load BTF if that is
   enabled, ignores the SHF_ALLOC flag. Since there's no need to support
   invalid section names, validate all of them, not just SHF_ALLOC
   sections.
2. Section names were validated *after* accessing them for the purposes
   of detecting ".modinfo" and ".gnu.linkonce.this_module". They are now
   checked prior to the access, which could avoid bad accesses with
   malformed modules.

Signed-off-by: Matthew Maurer <mmaurer@google.com>
---
 kernel/module/main.c | 106 ++++++++++++++++++++++++++++---------------
 1 file changed, 69 insertions(+), 37 deletions(-)

diff --git a/kernel/module/main.c b/kernel/module/main.c
index c480fd33861a..252cfa9eee67 100644
--- a/kernel/module/main.c
+++ b/kernel/module/main.c
@@ -1789,6 +1789,71 @@ static int elf_validity_cache_sechdrs(struct load_info *info)
 	return 0;
 }
 
+/**
+ * elf_validity_cache_secstrings() - Caches section names if valid
+ * @info: Load info to cache section names from. Must have valid sechdrs.
+ *
+ * Specifically checks:
+ *
+ * * Section name table index is inbounds of section headers
+ * * Section name table is not empty
+ * * Section name table is NUL terminated
+ * * All section name offsets are inbounds of the section
+ *
+ * Then updates @info with a &load_info->secstrings pointer if valid.
+ *
+ * Return: %0 if valid, negative error code if validation failed.
+ */
+static int elf_validity_cache_secstrings(struct load_info *info)
+{
+	Elf_Shdr *strhdr, *shdr;
+	char *secstrings;
+	int i;
+
+	/*
+	 * Verify if the section name table index is valid.
+	 */
+	if (info->hdr->e_shstrndx == SHN_UNDEF
+	    || info->hdr->e_shstrndx >= info->hdr->e_shnum) {
+		pr_err("Invalid ELF section name index: %d || e_shstrndx (%d) >= e_shnum (%d)\n",
+		       info->hdr->e_shstrndx, info->hdr->e_shstrndx,
+		       info->hdr->e_shnum);
+		return -ENOEXEC;
+	}
+
+	strhdr = &info->sechdrs[info->hdr->e_shstrndx];
+
+	/*
+	 * The section name table must be NUL-terminated, as required
+	 * by the spec. This makes strcmp and pr_* calls that access
+	 * strings in the section safe.
+	 */
+	secstrings = (void *)info->hdr + strhdr->sh_offset;
+	if (strhdr->sh_size == 0) {
+		pr_err("empty section name table\n");
+		return -ENOEXEC;
+	}
+	if (secstrings[strhdr->sh_size - 1] != '\0') {
+		pr_err("ELF Spec violation: section name table isn't null terminated\n");
+		return -ENOEXEC;
+	}
+
+	for (i = 0; i < info->hdr->e_shnum; i++) {
+		shdr = &info->sechdrs[i];
+		/* SHT_NULL means sh_name has an undefined value */
+		if (shdr->sh_type == SHT_NULL)
+			continue;
+		if (shdr->sh_name >= strhdr->sh_size) {
+			pr_err("Invalid ELF section name in module (section %u type %u)\n",
+			       i, shdr->sh_type);
+			return -ENOEXEC;
+		}
+	}
+
+	info->secstrings = secstrings;
+	return 0;
+}
+
 /*
  * Check userspace passed ELF module against our expectations, and cache
  * useful variables for further processing as we go.
@@ -1812,7 +1877,7 @@ static int elf_validity_cache_sechdrs(struct load_info *info)
 static int elf_validity_cache_copy(struct load_info *info, int flags)
 {
 	unsigned int i;
-	Elf_Shdr *shdr, *strhdr;
+	Elf_Shdr *shdr;
 	int err;
 	unsigned int num_mod_secs = 0, mod_idx;
 	unsigned int num_info_secs = 0, info_idx;
@@ -1821,34 +1886,9 @@ static int elf_validity_cache_copy(struct load_info *info, int flags)
 	err = elf_validity_cache_sechdrs(info);
 	if (err < 0)
 		return err;
-
-	/*
-	 * Verify if the section name table index is valid.
-	 */
-	if (info->hdr->e_shstrndx == SHN_UNDEF
-	    || info->hdr->e_shstrndx >= info->hdr->e_shnum) {
-		pr_err("Invalid ELF section name index: %d || e_shstrndx (%d) >= e_shnum (%d)\n",
-		       info->hdr->e_shstrndx, info->hdr->e_shstrndx,
-		       info->hdr->e_shnum);
-		goto no_exec;
-	}
-
-	strhdr = &info->sechdrs[info->hdr->e_shstrndx];
-
-	/*
-	 * The section name table must be NUL-terminated, as required
-	 * by the spec. This makes strcmp and pr_* calls that access
-	 * strings in the section safe.
-	 */
-	info->secstrings = (void *)info->hdr + strhdr->sh_offset;
-	if (strhdr->sh_size == 0) {
-		pr_err("empty section name table\n");
-		goto no_exec;
-	}
-	if (info->secstrings[strhdr->sh_size - 1] != '\0') {
-		pr_err("ELF Spec violation: section name table isn't null terminated\n");
-		goto no_exec;
-	}
+	err = elf_validity_cache_secstrings(info);
+	if (err < 0)
+		return err;
 
 	for (i = 1; i < info->hdr->e_shnum; i++) {
 		shdr = &info->sechdrs[i];
@@ -1877,14 +1917,6 @@ static int elf_validity_cache_copy(struct load_info *info, int flags)
 				num_info_secs++;
 				info_idx = i;
 			}
-
-			if (shdr->sh_flags & SHF_ALLOC) {
-				if (shdr->sh_name >= strhdr->sh_size) {
-					pr_err("Invalid ELF section name in module (section %u type %u)\n",
-					       i, shdr->sh_type);
-					return -ENOEXEC;
-				}
-			}
 			break;
 		}
 	}
-- 
2.46.0.rc2.264.g509ed76dc8-goog