[PATCH] coredump: Also dump first pages of non-executable ELF libraries

[Jann Horn]

When I rewrote the VMA dumping logic for coredumps, I changed it to
recognize ELF library mappings based on the file being executable instead
of the mapping having an ELF header. But turns out, distros ship many ELF
libraries as non-executable, so the heuristic goes wrong...

Restore the old behavior where FILTER(ELF_HEADERS) dumps the first page of
any offset-0 readable mapping that starts with the ELF magic.

This fix is technically layer-breaking a bit, because it checks for
something ELF-specific in fs/coredump.c; but since we probably want to
share this between standard ELF and FDPIC ELF anyway, I guess it's fine?
And this also keeps the change small for backporting.

Cc: stable@xxxxxxxxxxxxxxx
Fixes: 429a22e776a2 ("coredump: rework elf/elf_fdpic vma_dump_size() into common helper")
Reported-by: Bill Messmer <wmessmer@xxxxxxxxxxxxx>
Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
---

@Bill: If you happen to have a kernel tree lying around, you could give
this a try and report back whether this solves your issues?
But if not, it's also fine, I've tested myself that with this patch
applied, the first 0x1000 bytes of non-executable libraries are dumped
into the coredump according to "readelf".

 fs/coredump.c | 39 ++++++++++++++++++++++++++++++++++-----
 1 file changed, 34 insertions(+), 5 deletions(-)

diff --git a/fs/coredump.c b/fs/coredump.c
index 1c060c0a2d72..b73817712dd2 100644
--- a/fs/coredump.c
+++ b/fs/coredump.c
@@ -42,6 +42,7 @@
 #include <linux/path.h>
 #include <linux/timekeeping.h>
 #include <linux/sysctl.h>
+#include <linux/elf.h>
 
 #include <linux/uaccess.h>
 #include <asm/mmu_context.h>
@@ -980,6 +981,8 @@ static bool always_dump_vma(struct vm_area_struct *vma)
 	return false;
 }
 
+#define DUMP_SIZE_MAYBE_ELFHDR_PLACEHOLDER 1
+
 /*
  * Decide how much of @vma's contents should be included in a core dump.
  */
@@ -1039,9 +1042,20 @@ static unsigned long vma_dump_size(struct vm_area_struct *vma,
 	 * dump the first page to aid in determining what was mapped here.
 	 */
 	if (FILTER(ELF_HEADERS) &&
-	    vma->vm_pgoff == 0 && (vma->vm_flags & VM_READ) &&
-	    (READ_ONCE(file_inode(vma->vm_file)->i_mode) & 0111) != 0)
-		return PAGE_SIZE;
+	    vma->vm_pgoff == 0 && (vma->vm_flags & VM_READ)) {
+		if ((READ_ONCE(file_inode(vma->vm_file)->i_mode) & 0111) != 0)
+			return PAGE_SIZE;
+
+		/*
+		 * ELF libraries aren't always executable.
+		 * We'll want to check whether the mapping starts with the ELF
+		 * magic, but not now - we're holding the mmap lock,
+		 * so copy_from_user() doesn't work here.
+		 * Use a placeholder instead, and fix it up later in
+		 * dump_vma_snapshot().
+		 */
+		return DUMP_SIZE_MAYBE_ELFHDR_PLACEHOLDER;
+	}
 
 #undef	FILTER
 
@@ -1116,8 +1130,6 @@ int dump_vma_snapshot(struct coredump_params *cprm, int *vma_count,
 		m->end = vma->vm_end;
 		m->flags = vma->vm_flags;
 		m->dump_size = vma_dump_size(vma, cprm->mm_flags);
-
-		vma_data_size += m->dump_size;
 	}
 
 	mmap_write_unlock(mm);
@@ -1127,6 +1139,23 @@ int dump_vma_snapshot(struct coredump_params *cprm, int *vma_count,
 		return -EFAULT;
 	}
 
+	for (i = 0; i < *vma_count; i++) {
+		struct core_vma_metadata *m = (*vma_meta) + i;
+
+		if (m->dump_size == DUMP_SIZE_MAYBE_ELFHDR_PLACEHOLDER) {
+			char elfmag[SELFMAG];
+
+			if (copy_from_user(elfmag, (void __user *)m->start, SELFMAG) ||
+					memcmp(elfmag, ELFMAG, SELFMAG) != 0) {
+				m->dump_size = 0;
+			} else {
+				m->dump_size = PAGE_SIZE;
+			}
+		}
+
+		vma_data_size += m->dump_size;
+	}
+
 	*vma_data_size_ptr = vma_data_size;
 	return 0;
 }

base-commit: 0280e3c58f92b2fe0e8fbbdf8d386449168de4a8
-- 
2.35.0.rc0.227.g00780c9af4-goog