[PATCH] binfmt_elf: Reintroduce using MAP_FIXED_NOREPLACE

[Kees Cook]

Commit b212921b13bd ("elf: don't use MAP_FIXED_NOREPLACE for elf
executable mappings") reverted back to using MAP_FIXED to map ELF
LOAD segments because it was found that the segments in some binaries
overlap and can cause MAP_FIXED_NOREPLACE to fail. The original intent of
MAP_FIXED_NOREPLACE was to prevent the silent clobbering of an existing
mapping (e.g. stack) by the ELF image. To achieve this, expand on the
logic used when loading ET_DYN binaries which calculates a total size
for the image when the first segment is mapped, maps the entire image,
and then unmaps the remainder before remaining segments are mapped.
Apply this to ET_EXEC binaries as well as ET_DYN binaries as is done
now, and for both ET_EXEC and ET_DYN+INTERP use MAP_FIXED_NOREPLACE for
the initial total size mapping and MAP_FIXED for remaining mappings.
For ET_DYN w/out INTERP, continue to map at a system-selected address
in the mmap region.

Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Russell King <linux@xxxxxxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Eric Biederman <ebiederm@xxxxxxxxxxxx>
Co-developed-by: Anthony Yznaga <anthony.yznaga@xxxxxxxxxx>
Signed-off-by: Anthony Yznaga <anthony.yznaga@xxxxxxxxxx>
Link: https://lore.kernel.org/lkml/1595869887-23307-2-git-send-email-anthony.yznaga@xxxxxxxxxx
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 fs/binfmt_elf.c | 31 ++++++++++++++++++++++---------
 1 file changed, 22 insertions(+), 9 deletions(-)

diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index 439ed81e755a..ef00bf8bd6f4 100644
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -1074,20 +1074,26 @@ static int load_elf_binary(struct linux_binprm *bprm)
 
 		vaddr = elf_ppnt->p_vaddr;
 		/*
-		 * If we are loading ET_EXEC or we have already performed
-		 * the ET_DYN load_addr calculations, proceed normally.
+		 * The first time through the loop, load_addr_set is false:
+		 * layout will be calculated. Once set, use MAP_FIXED since
+		 * we know we've already safely mapped the entire region with
+		 * MAP_FIXED_NOREPLACE in the once-per-binary logic following.
 		 */
-		if (elf_ex->e_type == ET_EXEC || load_addr_set) {
+		if (load_addr_set) {
 			elf_flags |= MAP_FIXED;
+		} else if (elf_ex->e_type == ET_EXEC) {
+			/*
+			 * This logic is run once for the first LOAD Program
+			 * Header for ET_EXEC binaries. No special handling
+			 * is needed.
+			 */
+			elf_flags |= MAP_FIXED_NOREPLACE;
 		} else if (elf_ex->e_type == ET_DYN) {
 			/*
 			 * This logic is run once for the first LOAD Program
 			 * Header for ET_DYN binaries to calculate the
 			 * randomization (load_bias) for all the LOAD
-			 * Program Headers, and to calculate the entire
-			 * size of the ELF mapping (total_size). (Note that
-			 * load_addr_set is set to true later once the
-			 * initial mapping is performed.)
+			 * Program Headers.
 			 *
 			 * There are effectively two types of ET_DYN
 			 * binaries: programs (i.e. PIE: ET_DYN with INTERP)
@@ -1108,7 +1114,7 @@ static int load_elf_binary(struct linux_binprm *bprm)
 			 * Therefore, programs are loaded offset from
 			 * ELF_ET_DYN_BASE and loaders are loaded into the
 			 * independently randomized mmap region (0 load_bias
-			 * without MAP_FIXED).
+			 * without MAP_FIXED nor MAP_FIXED_NOREPLACE).
 			 */
 			if (interpreter) {
 				load_bias = ELF_ET_DYN_BASE;
@@ -1117,7 +1123,7 @@ static int load_elf_binary(struct linux_binprm *bprm)
 				alignment = maximum_alignment(elf_phdata, elf_ex->e_phnum);
 				if (alignment)
 					load_bias &= ~(alignment - 1);
-				elf_flags |= MAP_FIXED;
+				elf_flags |= MAP_FIXED_NOREPLACE;
 			} else
 				load_bias = 0;
 
@@ -1129,7 +1135,14 @@ static int load_elf_binary(struct linux_binprm *bprm)
 			 * is then page aligned.
 			 */
 			load_bias = ELF_PAGESTART(load_bias - vaddr);
+		}
 
+		/*
+		 * Calculate the entire size of the ELF mapping (total_size).
+		 * (Note that load_addr_set is set to true later once the
+		 * initial mapping is performed.)
+		 */
+		if (!load_addr_set) {
 			total_size = total_mapping_size(elf_phdata,
 							elf_ex->e_phnum);
 			if (!total_size) {
-- 
2.30.2