From 3fb4afd9a504c2386b8435028d43283216bf588e Mon Sep 17 00:00:00 2001 From: Stanislav Kinsburskiy Date: Mon, 12 Dec 2016 16:40:42 -0800 Subject: [PATCH] prctl: remove one-shot limitation for changing exe link This limitation came with the reason to remove "another way for malicious code to obscure a compromised program and masquerade as a benign process" by allowing "security-concious program can use this prctl once during its early initialization to ensure the prctl cannot later be abused for this purpose": http://marc.info/?l=linux-kernel&m=133160684517468&w=2 This explanation doesn't look sufficient. The only thing "exe" link is indicating is the file, used to execve, which is basically nothing and not reliable immediately after process has returned from execve system call. Moreover, to use this feture, all the mappings to previous exe file have to be unmapped and all the new exe file permissions must be satisfied. Which means, that changing exe link is very similar to calling execve on the binary. The need to remove this limitations comes from migration of NFS mount point, which is not accessible during restore and replaced by other file system. Because of this exe link has to be changed twice. [akpm@linux-foundation.org: fix up comment] Link: http://lkml.kernel.org/r/20160927153755.9337.69650.stgit@localhost.localdomain Signed-off-by: Stanislav Kinsburskiy Acked-by: Oleg Nesterov Acked-by: Cyrill Gorcunov Cc: Peter Zijlstra Cc: Ingo Molnar Cc: Michal Hocko Cc: Kees Cook Cc: Andy Lutomirski Cc: John Stultz Cc: Matt Helsley Cc: Pavel Emelyanov Cc: Vlastimil Babka Cc: Eric W. Biederman Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- include/linux/sched.h | 6 +++++- kernel/sys.c | 10 ---------- 2 files changed, 5 insertions(+), 11 deletions(-) diff --git a/include/linux/sched.h b/include/linux/sched.h index 7551d3e..0e90f29 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -540,7 +540,11 @@ static inline int get_dumpable(struct mm_struct *mm) /* leave room for more dump flags */ #define MMF_VM_MERGEABLE 16 /* KSM may merge identical pages */ #define MMF_VM_HUGEPAGE 17 /* set when VM_HUGEPAGE is set on vma */ -#define MMF_EXE_FILE_CHANGED 18 /* see prctl_set_mm_exe_file() */ +/* + * This one-shot flag is dropped due to necessity of changing exe once again + * on NFS restore + */ +//#define MMF_EXE_FILE_CHANGED 18 /* see prctl_set_mm_exe_file() */ #define MMF_HAS_UPROBES 19 /* has uprobes */ #define MMF_RECALC_UPROBES 20 /* MMF_HAS_UPROBES can be wrong */ diff --git a/kernel/sys.c b/kernel/sys.c index 89d5be4..fd6f508 100644 --- a/kernel/sys.c +++ b/kernel/sys.c @@ -1696,16 +1696,6 @@ static int prctl_set_mm_exe_file(struct mm_struct *mm, unsigned int fd) fput(exe_file); } - /* - * The symlink can be changed only once, just to disallow arbitrary - * transitions malicious software might bring in. This means one - * could make a snapshot over all processes running and monitor - * /proc/pid/exe changes to notice unusual activity if needed. - */ - err = -EPERM; - if (test_and_set_bit(MMF_EXE_FILE_CHANGED, &mm->flags)) - goto exit; - err = 0; /* set the new file, lockless */ get_file(exe.file); -- 2.7.4