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tac: remove MMAP use of untrusted source #9748 #9772

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tac: remove MMAP use of untrusted source as truncation results in cor…
cerdelen Dec 22, 2025
6b3abce
Merge branch 'main' into main
sylvestre Dec 29, 2025
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65 changes: 17 additions & 48 deletions src/uu/tac/src/tac.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -8,13 +8,9 @@ mod error;

use clap::{Arg, ArgAction, Command};
use memchr::memmem;
use memmap2::Mmap;
use std::ffi::OsString;
use std::io::{BufWriter, Read, Write, stdin, stdout};
use std::{
fs::{File, read},
path::Path,
};
use std::{fs::read, path::Path};
use uucore::error::UError;
use uucore::error::UResult;
use uucore::{format_usage, show};
Expand Down Expand Up @@ -233,23 +229,17 @@ fn tac(filenames: &[OsString], before: bool, regex: bool, separator: &str) -> UR
};

for filename in filenames {
let mmap;
let buf;

let data: &[u8] = if filename == "-" {
if let Some(mmap1) = try_mmap_stdin() {
mmap = mmap1;
&mmap
} else {
let mut buf1 = Vec::new();
if let Err(e) = stdin().read_to_end(&mut buf1) {
let e: Box<dyn UError> = TacError::ReadError(OsString::from("stdin"), e).into();
show!(e);
continue;
}
buf = buf1;
&buf
let mut buf1 = Vec::new();
if let Err(e) = stdin().read_to_end(&mut buf1) {
let e: Box<dyn UError> = TacError::ReadError(OsString::from("stdin"), e).into();
show!(e);
continue;
}
buf = buf1;
&buf
} else {
let path = Path::new(filename);
if path.is_dir() {
Expand All @@ -264,20 +254,15 @@ fn tac(filenames: &[OsString], before: bool, regex: bool, separator: &str) -> UR
continue;
}

if let Some(mmap1) = try_mmap_path(path) {
mmap = mmap1;
&mmap
} else {
match read(path) {
Ok(buf1) => {
buf = buf1;
&buf
}
Err(e) => {
let e: Box<dyn UError> = TacError::ReadError(filename.clone(), e).into();
show!(e);
continue;
}
match read(path) {
Ok(buf1) => {
buf = buf1;
&buf
}
Err(e) => {
let e: Box<dyn UError> = TacError::ReadError(filename.clone(), e).into();
show!(e);
continue;
}
}
};
Expand All @@ -296,19 +281,3 @@ fn tac(filenames: &[OsString], before: bool, regex: bool, separator: &str) -> UR
}
Ok(())
}

fn try_mmap_stdin() -> Option<Mmap> {
// SAFETY: If the file is truncated while we map it, SIGBUS will be raised
// and our process will be terminated, thus preventing access of invalid memory.
unsafe { Mmap::map(&stdin()).ok() }
}

fn try_mmap_path(path: &Path) -> Option<Mmap> {
let file = File::open(path).ok()?;

// SAFETY: If the file is truncated while we map it, SIGBUS will be raised
// and our process will be terminated, thus preventing access of invalid memory.
let mmap = unsafe { Mmap::map(&file).ok()? };

Some(mmap)
}
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