Re: [PATCH] ovl: fix mmap denywrite
From: Christian König <christian.koenig@amd.com>
Date: 2021-06-22 12:43:52
Also in:
linux-fsdevel, linux-mm
Am 22.06.21 um 14:30 schrieb Miklos Szeredi:
Overlayfs did not honor positive i_writecount on realfile for VM_DENYWRITE mappings. Similarly negative i_mmap_writable counts were ignored for VM_SHARED mappings. Fix by making vma_set_file() switch the temporary counts obtained and released by mmap_region().
Mhm, I don't fully understand the background but that looks like something specific to overlayfs to me. So why are you changing the common helper? Thanks, Christian.
quoted hunk ↗ jump to hunk
Reported-by: Chengguang Xu <redacted> Signed-off-by: Miklos Szeredi <redacted> --- fs/overlayfs/file.c | 4 +++- include/linux/mm.h | 1 + mm/mmap.c | 2 +- mm/util.c | 38 +++++++++++++++++++++++++++++++++++++- 4 files changed, 42 insertions(+), 3 deletions(-)--- a/fs/overlayfs/file.c +++ b/fs/overlayfs/file.c@@ -430,7 +430,9 @@ static int ovl_mmap(struct file *file, s if (WARN_ON(file != vma->vm_file)) return -EIO; - vma_set_file(vma, realfile); + ret = vma_set_file_checkwrite(vma, realfile); + if (ret) + return ret; old_cred = ovl_override_creds(file_inode(file)->i_sb); ret = call_mmap(vma->vm_file, vma); --- a/include/linux/mm.h +++ b/include/linux/mm.h@@ -2751,6 +2751,7 @@ static inline void vma_set_page_prot(str #endif void vma_set_file(struct vm_area_struct *vma, struct file *file); +int vma_set_file_checkwrite(struct vm_area_struct *vma, struct file *file); #ifdef CONFIG_NUMA_BALANCING unsigned long change_prot_numa(struct vm_area_struct *vma, --- a/mm/mmap.c +++ b/mm/mmap.c@@ -1809,6 +1809,7 @@ unsigned long mmap_region(struct file *f */ vma->vm_file = get_file(file); error = call_mmap(file, vma); + file = vma->vm_file;
quoted hunk ↗ jump to hunk
if (error) goto unmap_and_free_vma;@@ -1870,7 +1871,6 @@ unsigned long mmap_region(struct file *f if (vm_flags & VM_DENYWRITE) allow_write_access(file); } - file = vma->vm_file; out: perf_event_mmap(vma); --- a/mm/util.c +++ b/mm/util.c@@ -314,12 +314,48 @@ int vma_is_stack_for_current(struct vm_a /* * Change backing file, only valid to use during initial VMA setup. */ -void vma_set_file(struct vm_area_struct *vma, struct file *file) +int vma_set_file_checkwrite(struct vm_area_struct *vma, struct file *file) { + vm_flags_t vm_flags = vma->vm_flags; + int err = 0; + /* Changing an anonymous vma with this is illegal */ get_file(file); + + /* Get temporary denial counts on replacement */ + if (vm_flags & VM_DENYWRITE) { + err = deny_write_access(file); + if (err) + goto out_put; + } + if (vm_flags & VM_SHARED) { + err = mapping_map_writable(file->f_mapping); + if (err) + goto out_allow; + } + swap(vma->vm_file, file); + + /* Undo temporary denial counts on replaced */ + if (vm_flags & VM_SHARED) + mapping_unmap_writable(file->f_mapping); +out_allow: + if (vm_flags & VM_DENYWRITE) + allow_write_access(file); +out_put: fput(file); + return err; +} +EXPORT_SYMBOL(vma_set_file_checkwrite); + +/* + * Change backing file, only valid to use during initial VMA setup. + */ +void vma_set_file(struct vm_area_struct *vma, struct file *file) +{ + int err = vma_set_file_checkwrite(vma, file); + + WARN_ON_ONCE(err); } EXPORT_SYMBOL(vma_set_file);