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@xxxxxxxxxxxx> > > [ 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@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 > Tested-by: Pedro Falcato <pedro.falcato@xxxxxxxxx> > Signed-off-by: Sebastian Ott <sebott@xxxxxxxxxx> > Link: https://lore.kernel.org/r/20230929032435.2391507-1-keescook@xxxxxxxxxxxx > Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx> > Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx> > --- > 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_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, > -- > 2.42.0 > -- Kees Cook
