From b6b7a8faf05c709cd9f63d3b7d9c66bd91bc3b0d Mon Sep 17 00:00:00 2001 From: David Hildenbrand Date: Mon, 2 Jan 2023 17:08:55 +0100 Subject: [PATCH] mm/nommu: don't use VM_MAYSHARE for MAP_PRIVATE mappings Let's stop using VM_MAYSHARE for MAP_PRIVATE mappings and use VM_MAYOVERLAY instead. Rewrite determine_vm_flags() to make the whole logic easier to digest, and to cleanly separate MAP_PRIVATE vs. MAP_SHARED. No functional change intended. Link: https://lkml.kernel.org/r/20230102160856.500584-3-david@redhat.com Signed-off-by: David Hildenbrand Cc: Arnd Bergmann Cc: Greg Kroah-Hartman Cc: Jens Axboe Cc: Nicolas Pitre Cc: Pavel Begunkov Signed-off-by: Andrew Morton --- include/linux/mm.h | 7 ++++++- mm/nommu.c | 51 ++++++++++++++++++++++++++++++--------------------- 2 files changed, 36 insertions(+), 22 deletions(-) diff --git a/include/linux/mm.h b/include/linux/mm.h index 791bac4..8a85633 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -276,7 +276,12 @@ extern unsigned int kobjsize(const void *objp); #define VM_MAYSHARE 0x00000080 #define VM_GROWSDOWN 0x00000100 /* general info on the segment */ +#ifdef CONFIG_MMU #define VM_UFFD_MISSING 0x00000200 /* missing pages tracking */ +#else /* CONFIG_MMU */ +#define VM_MAYOVERLAY 0x00000200 /* nommu: R/O MAP_PRIVATE mapping that might overlay a file mapping */ +#define VM_UFFD_MISSING 0 +#endif /* CONFIG_MMU */ #define VM_PFNMAP 0x00000400 /* Page-ranges managed without "struct page", just pure PFN */ #define VM_UFFD_WP 0x00001000 /* wrprotect pages tracking */ @@ -1358,7 +1363,7 @@ static inline bool is_nommu_shared_mapping(vm_flags_t flags) * ptrace does not apply. Note that there is no mprotect() to upgrade * write permissions later. */ - return flags & VM_MAYSHARE; + return flags & (VM_MAYSHARE | VM_MAYOVERLAY); } #endif diff --git a/mm/nommu.c b/mm/nommu.c index 1671ebb..df1711a 100644 --- a/mm/nommu.c +++ b/mm/nommu.c @@ -892,29 +892,36 @@ static unsigned long determine_vm_flags(struct file *file, unsigned long vm_flags; vm_flags = calc_vm_prot_bits(prot, 0) | calc_vm_flag_bits(flags); - /* vm_flags |= mm->def_flags; */ - if (!(capabilities & NOMMU_MAP_DIRECT)) { - /* attempt to share read-only copies of mapped file chunks */ + if (!file) { + /* + * MAP_ANONYMOUS. MAP_SHARED is mapped to MAP_PRIVATE, because + * there is no fork(). + */ vm_flags |= VM_MAYREAD | VM_MAYWRITE | VM_MAYEXEC; - if (file && !(prot & PROT_WRITE)) - vm_flags |= VM_MAYSHARE; + } else if (flags & MAP_PRIVATE) { + /* MAP_PRIVATE file mapping */ + if (capabilities & NOMMU_MAP_DIRECT) + vm_flags |= (capabilities & NOMMU_VMFLAGS); + else + vm_flags |= VM_MAYREAD | VM_MAYWRITE | VM_MAYEXEC; + + if (!(prot & PROT_WRITE) && !current->ptrace) + /* + * R/O private file mapping which cannot be used to + * modify memory, especially also not via active ptrace + * (e.g., set breakpoints) or later by upgrading + * permissions (no mprotect()). We can try overlaying + * the file mapping, which will work e.g., on chardevs, + * ramfs/tmpfs/shmfs and romfs/cramf. + */ + vm_flags |= VM_MAYOVERLAY; } else { - /* overlay a shareable mapping on the backing device or inode - * if possible - used for chardevs, ramfs/tmpfs/shmfs and - * romfs/cramfs */ - vm_flags |= VM_MAYSHARE | (capabilities & NOMMU_VMFLAGS); - if (flags & MAP_SHARED) - vm_flags |= VM_SHARED; + /* MAP_SHARED file mapping: NOMMU_MAP_DIRECT is set. */ + vm_flags |= VM_SHARED | VM_MAYSHARE | + (capabilities & NOMMU_VMFLAGS); } - /* refuse to let anyone share private mappings with this process if - * it's being traced - otherwise breakpoints set in it may interfere - * with another untraced process - */ - if ((flags & MAP_PRIVATE) && current->ptrace) - vm_flags &= ~VM_MAYSHARE; - return vm_flags; } @@ -952,9 +959,11 @@ static int do_mmap_private(struct vm_area_struct *vma, void *base; int ret, order; - /* invoke the file's mapping function so that it can keep track of - * shared mappings on devices or memory - * - VM_MAYSHARE will be set if it may attempt to share + /* + * Invoke the file's mapping function so that it can keep track of + * shared mappings on devices or memory. VM_MAYOVERLAY will be set if + * it may attempt to share, which will make is_nommu_shared_mapping() + * happy. */ if (capabilities & NOMMU_MAP_DIRECT) { ret = call_mmap(vma->vm_file, vma); -- 2.7.4