From: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx> 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@xxxxxx> > 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@xxxxxxx> > Signed-off-by: Andrew Morton <akpm@xxxxxxxx> > Signed-off-by: Linus Torvalds <torvalds@xxxxxxxx> 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@xxxxxxxxxx> Reported-by: Thomas Weißschuh <linux@xxxxxxxxxxxxxx> Closes: https://lkml.kernel.org/r/20230914-bss-alloc-v1-1-78de67d2c6dd@xxxxxxxxxxxxxx Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx> Link: https://lore.kernel.org/r/87sf71f123.fsf@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx> --- 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 7b3d2d491407..2a615f476e44 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_filesz - before 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); if (BAD_ADDR(error)) { retval = IS_ERR_VALUE(error) ? @@ -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, -- 2.34.1
