Please drop this from -stable -- it's part of a larger refactoring that shouldn't be backported without explicit effort/testing.
-Kees
On Mon, Nov 06, 2023 at 06:14:18PM -0500, Sasha Levin wrote:
From: "Eric W. Biederman" ebiederm@xmission.com
[ Upstream commit 585a018627b4d7ed37387211f667916840b5c5ea ]
Implement a helper elf_load() that wraps elf_map() and performs all of the necessary work to ensure that when "memsz > filesz" the bytes described by "memsz > filesz" are zeroed.
An outstanding issue is if the first segment has filesz 0, and has a randomized location. But that is the same as today.
In this change I replaced an open coded padzero() that did not clear all of the way to the end of the page, with padzero() that does.
I also stopped checking the return of padzero() as there is at least one known case where testing for failure is the wrong thing to do. It looks like binfmt_elf_fdpic may have the proper set of tests for when error handling can be safely completed.
I found a couple of commits in the old history https://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git, that look very interesting in understanding this code.
commit 39b56d902bf3 ("[PATCH] binfmt_elf: clearing bss may fail") commit c6e2227e4a3e ("[SPARC64]: Missing user access return value checks in fs/binfmt_elf.c and fs/compat.c") commit 5bf3be033f50 ("v2.4.10.1 -> v2.4.10.2")
Looking at commit 39b56d902bf3 ("[PATCH] binfmt_elf: clearing bss may fail"):
commit 39b56d902bf35241e7cba6cc30b828ed937175ad Author: Pavel Machek pavel@ucw.cz Date: Wed Feb 9 22:40:30 2005 -0800
[PATCH] binfmt_elf: clearing bss may fail So we discover that Borland's Kylix application builder emits weird elf files which describe a non-writeable bss segment. So remove the clear_user() check at the place where we zero out the bss. I don't _think_ there are any security implications here (plus we've never checked that clear_user() return value, so whoops if it is a problem). Signed-off-by: Pavel Machek <pavel@suse.cz> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>It seems pretty clear that binfmt_elf_fdpic with skipping clear_user() for non-writable segments and otherwise calling clear_user(), aka padzero(), and checking it's return code is the right thing to do.
I just skipped the error checking as that avoids breaking things.
And notably, it looks like Borland's Kylix died in 2005 so it might be safe to just consider read-only segments with memsz > filesz an error.
Reported-by: Sebastian Ott sebott@redhat.com Reported-by: Thomas Weißschuh linux@weissschuh.net Closes: https://lkml.kernel.org/r/20230914-bss-alloc-v1-1-78de67d2c6dd@weissschuh.ne... Signed-off-by: "Eric W. Biederman" ebiederm@xmission.com Link: https://lore.kernel.org/r/87sf71f123.fsf@email.froward.int.ebiederm.org Tested-by: Pedro Falcato pedro.falcato@gmail.com Signed-off-by: Sebastian Ott sebott@redhat.com Link: https://lore.kernel.org/r/20230929032435.2391507-1-keescook@chromium.org Signed-off-by: Kees Cook keescook@chromium.org Signed-off-by: Sasha Levin sashal@kernel.org
fs/binfmt_elf.c | 111 +++++++++++++++++++++--------------------------- 1 file changed, 48 insertions(+), 63 deletions(-)
diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c index 7b3d2d4914073..2a615f476e44e 100644 --- a/fs/binfmt_elf.c +++ b/fs/binfmt_elf.c @@ -110,25 +110,6 @@ static struct linux_binfmt elf_format = { #define BAD_ADDR(x) (unlikely((unsigned long)(x) >= TASK_SIZE)) -static int set_brk(unsigned long start, unsigned long end, int prot) -{
- start = ELF_PAGEALIGN(start);
- end = ELF_PAGEALIGN(end);
- if (end > start) {
/** Map the last of the bss segment.* If the header is requesting these pages to be* executable, honour that (ppc32 needs this).*/int error = vm_brk_flags(start, end - start,prot & PROT_EXEC ? VM_EXEC : 0);if (error)return error;- }
- current->mm->start_brk = current->mm->brk = end;
- return 0;
-}
/* We need to explicitly zero any fractional pages after the data section (i.e. bss). This would contain the junk from the file that should not @@ -406,6 +387,51 @@ static unsigned long elf_map(struct file *filep, unsigned long addr, return(map_addr); } +static unsigned long elf_load(struct file *filep, unsigned long addr,
const struct elf_phdr *eppnt, int prot, int type,unsigned long total_size)+{
- unsigned long zero_start, zero_end;
- unsigned long map_addr;
- if (eppnt->p_filesz) {
map_addr = elf_map(filep, addr, eppnt, prot, type, total_size);if (BAD_ADDR(map_addr))return map_addr;if (eppnt->p_memsz > eppnt->p_filesz) {zero_start = map_addr + ELF_PAGEOFFSET(eppnt->p_vaddr) +eppnt->p_filesz;zero_end = map_addr + ELF_PAGEOFFSET(eppnt->p_vaddr) +eppnt->p_memsz;/* Zero the end of the last mapped page */padzero(zero_start);}- } else {
map_addr = zero_start = ELF_PAGESTART(addr);zero_end = zero_start + ELF_PAGEOFFSET(eppnt->p_vaddr) +eppnt->p_memsz;- }
- if (eppnt->p_memsz > eppnt->p_filesz) {
/** Map the last of the segment.* If the header is requesting these pages to be* executable, honour that (ppc32 needs this).*/int error;zero_start = ELF_PAGEALIGN(zero_start);zero_end = ELF_PAGEALIGN(zero_end);error = vm_brk_flags(zero_start, zero_end - zero_start,prot & PROT_EXEC ? VM_EXEC : 0);if (error)map_addr = error;- }
- return map_addr;
+}
static unsigned long total_mapping_size(const struct elf_phdr *phdr, int nr) { elf_addr_t min_addr = -1; @@ -829,7 +855,6 @@ static int load_elf_binary(struct linux_binprm *bprm) struct elf_phdr *elf_ppnt, *elf_phdata, *interp_elf_phdata = NULL; struct elf_phdr *elf_property_phdata = NULL; unsigned long elf_bss, elf_brk;
- int bss_prot = 0; int retval, i; unsigned long elf_entry; unsigned long e_entry;
@@ -1040,33 +1065,6 @@ static int load_elf_binary(struct linux_binprm *bprm) if (elf_ppnt->p_type != PT_LOAD) continue;
if (unlikely (elf_brk > elf_bss)) {unsigned long nbyte;/* There was a PT_LOAD segment with p_memsz > p_fileszbefore this one. Map anonymous pages, if needed,and clear the area. */retval = set_brk(elf_bss + load_bias,elf_brk + load_bias,bss_prot);if (retval)goto out_free_dentry;nbyte = ELF_PAGEOFFSET(elf_bss);if (nbyte) {nbyte = ELF_MIN_ALIGN - nbyte;if (nbyte > elf_brk - elf_bss)nbyte = elf_brk - elf_bss;if (clear_user((void __user *)elf_bss +load_bias, nbyte)) {/** This bss-zeroing can fail if the ELF* file specifies odd protections. So* we don't check the return value*/}}}- elf_prot = make_prot(elf_ppnt->p_flags, &arch_state, !!interpreter, false);
@@ -1162,7 +1160,7 @@ static int load_elf_binary(struct linux_binprm *bprm) } }
error = elf_map(bprm->file, load_bias + vaddr, elf_ppnt,
error = elf_load(bprm->file, load_bias + vaddr, elf_ppnt, elf_prot, elf_flags, total_size);@@ -1217,10 +1215,8 @@ static int load_elf_binary(struct linux_binprm *bprm) if (end_data < k) end_data = k; k = elf_ppnt->p_vaddr + elf_ppnt->p_memsz;
if (k > elf_brk) {bss_prot = elf_prot;
if (k > elf_brk) elf_brk = k;
}e_entry = elf_ex->e_entry + load_bias; @@ -1232,18 +1228,7 @@ static int load_elf_binary(struct linux_binprm *bprm) start_data += load_bias; end_data += load_bias;
- /* Calling set_brk effectively mmaps the pages that we need
* for the bss and break sections. We must do this before* mapping in the interpreter, to make sure it doesn't wind* up getting placed where the bss needs to go.*/- retval = set_brk(elf_bss, elf_brk, bss_prot);
- if (retval)
goto out_free_dentry;- if (likely(elf_bss != elf_brk) && unlikely(padzero(elf_bss))) {
retval = -EFAULT; /* Nobody gets to see this, but.. */goto out_free_dentry;- }
- current->mm->start_brk = current->mm->brk = ELF_PAGEALIGN(elf_brk);
if (interpreter) { elf_entry = load_elf_interp(interp_elf_ex, -- 2.42.0